Explicitly use bundler 2.7 on Ruby 4 CI

Fixes a CI failure that can be seen on these CI runs:
* https://github.com/projectblacklight/blacklight/actions/runs/20819455247/job/59803659992
* https://github.com/projectblacklight/blacklight/actions/runs/20789330303/job/59774334565

When we did not specify a bundler version, we got this message in
the CI logs:

```
Using Bundler shipped with ruby-4.0.0
```

Then this one:

```
       rails  importmap:install
/opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:356:in 'Bundler::Resolver#raise_not_found!': Could not find gems matching 'sqlite3 (>= 2.1)' valid for all resolution platforms (aarch64-linux-gnu, aarch64-linux-musl, arm-linux-gnu, arm-linux-musl, arm64-darwin, x86_64-darwin, x86_64-linux-gnu, x86_64-linux-musl, x86_64-linux) in locally installed gems. (Bundler::GemNotFound)

The source contains the following gems matching 'sqlite3 (>= 2.1)':
  * sqlite3-2.9.0-x86_64-linux-gnu
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:450:in 'block in Bundler::Resolver#prepare_dependencies'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:425:in 'Hash#each'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:425:in 'Enumerable#filter_map'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:425:in 'Bundler::Resolver#prepare_dependencies'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:63:in 'Bundler::Resolver#setup_solver'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:28:in 'Bundler::Resolver#start'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/definition.rb:755:in 'Bundler::Definition#start_resolution'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/definition.rb:350:in 'Bundler::Definition#resolve'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/definition.rb:668:in 'Bundler::Definition#materialize'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/definition.rb:240:in 'Bundler::Definition#specs'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/definition.rb:312:in 'Bundler::Definition#specs_for'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/runtime.rb:18:in 'Bundler::Runtime#setup'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler.rb:165:in 'Bundler.setup'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/setup.rb:32:in 'block in <top (required)>'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/ui/shell.rb:177:in 'Bundler::UI::Shell#with_level'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/ui/shell.rb:122:in 'Bundler::UI::Shell#silence'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/setup.rb:32:in '<top (required)>'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/rubygems.rb:1454:in 'Kernel#require'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/rubygems.rb:1454:in '<top (required)>'
	from <internal:gem_prelude>:2:in '<internal:gem_prelude>'
       rails  turbo:install stimulus:install
/opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:356:in 'Bundler::Resolver#raise_not_found!': Could not find gems matching 'sqlite3 (>= 2.1)' valid for all resolution platforms (aarch64-linux-gnu, aarch64-linux-musl, arm-linux-gnu, arm-linux-musl, arm64-darwin, x86_64-darwin, x86_64-linux-gnu, x86_64-linux-musl, x86_64-linux) in locally installed gems. (Bundler::GemNotFound)

The source contains the following gems matching 'sqlite3 (>= 2.1)':
  * sqlite3-2.9.0-x86_64-linux-gnu
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:450:in 'block in Bundler::Resolver#prepare_dependencies'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:425:in 'Hash#each'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:425:in 'Enumerable#filter_map'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:425:in 'Bundler::Resolver#prepare_dependencies'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:63:in 'Bundler::Resolver#setup_solver'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:28:in 'Bundler::Resolver#start'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/definition.rb:755:in 'Bundler::Definition#start_resolution'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/definition.rb:350:in 'Bundler::Definition#resolve'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/definition.rb:668:in 'Bundler::Definition#materialize'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/definition.rb:240:in 'Bundler::Definition#specs'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/definition.rb:312:in 'Bundler::Definition#specs_for'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/runtime.rb:18:in 'Bundler::Runtime#setup'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler.rb:165:in 'Bundler.setup'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/setup.rb:32:in 'block in <top (required)>'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/ui/shell.rb:177:in 'Bundler::UI::Shell#with_level'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/ui/shell.rb:122:in 'Bundler::UI::Shell#silence'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/setup.rb:32:in '<top (required)>'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/rubygems.rb:1454:in 'Kernel#require'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/rubygems.rb:1454:in '<top (required)>'
	from <internal:gem_prelude>:2:in '<internal:gem_prelude>'
       rails  css:install:bootstrap
/opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:356:in 'Bundler::Resolver#raise_not_found!': Could not find gems matching 'sqlite3 (>= 2.1)' valid for all resolution platforms (aarch64-linux-gnu, aarch64-linux-musl, arm-linux-gnu, arm-linux-musl, arm64-darwin, x86_64-darwin, x86_64-linux-gnu, x86_64-linux-musl, x86_64-linux) in locally installed gems. (Bundler::GemNotFound)

The source contains the following gems matching 'sqlite3 (>= 2.1)':
  * sqlite3-2.9.0-x86_64-linux-gnu
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:450:in 'block in Bundler::Resolver#prepare_dependencies'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:425:in 'Hash#each'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:425:in 'Enumerable#filter_map'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:425:in 'Bundler::Resolver#prepare_dependencies'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:63:in 'Bundler::Resolver#setup_solver'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/resolver.rb:28:in 'Bundler::Resolver#start'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/definition.rb:755:in 'Bundler::Definition#start_resolution'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/definition.rb:350:in 'Bundler::Definition#resolve'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/definition.rb:668:in 'Bundler::Definition#materialize'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/definition.rb:240:in 'Bundler::Definition#specs'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/definition.rb:312:in 'Bundler::Definition#specs_for'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/runtime.rb:18:in 'Bundler::Runtime#setup'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler.rb:165:in 'Bundler.setup'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/setup.rb:32:in 'block in <top (required)>'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/ui/shell.rb:177:in 'Bundler::UI::Shell#with_level'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/ui/shell.rb:122:in 'Bundler::UI::Shell#silence'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/bundler/setup.rb:32:in '<top (required)>'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/rubygems.rb:1454:in 'Kernel#require'
	from /opt/hostedtoolcache/Ruby/4.0.0/x64/lib/ruby/4.0.0/rubygems.rb:1454:in '<top (required)>'
	from <internal:gem_prelude>:2:in '<internal:gem_prelude>'
```

...and then the generated app did not have any CSS or JS, causing test failures.

This commit takes a two-pronged approach: it installs bundler 2.7.2, and then uses the `BUNDLER_VERSION`
envvar to make sure that we use it when generating the internal test app.

Author: sandbergja

.github/matrix.json

@@ -2,6 +2,7 @@
   "ruby": ["3.4"],
   "rails_version": ["8.1.1"],
   "bootstrap_version": [null],
+  "bundler_version": ["2.7.2"],
   "view_component_version": ["~> 4.0"],
   "api": [null],
   "additional_engine_cart_rails_options": ["--css=bootstrap"],

.github/workflows/test.yml

@@ -36,6 +36,7 @@ jobs:
       BOOTSTRAP_VERSION: ${{ matrix.bootstrap_version }}
       BLACKLIGHT_API_TEST: ${{ matrix.api }}
       ENGINE_CART_RAILS_OPTIONS: "--skip-git --skip-listen --skip-spring --skip-keeps --skip-kamal --skip-solid --skip-coffee --skip-test ${{ matrix.additional_engine_cart_rails_options }}"
+      BUNDLER_VERSION: ${{ matrix.bundler_version || '2.7.2' }}
     steps:
       - uses: actions/checkout@v4
         with:
@@ -44,6 +45,7 @@ jobs:
         uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}
+          bundler: ${{ matrix.bundler_version || '2.7.2' }}
       - name: Change permissions
         run: "chmod -f -R o-w /opt/hostedtoolcache/Ruby/3.2.5/x64/lib/ruby/gems/3.2.0/gems | :"
       - name: Install dependencies