Remove redundant bundler installation from GitHub Actions workflow

Before:
- name: Install dependencies
  run: gem install bundler rake

After:
- name: Install dependencies
  run: gem install rake

.github/workflows/linux.yml

@@ -23,6 +23,6 @@ jobs:
       env:
         CI: true
       run: |
-        gem install bundler rake
+        gem install rake
         bundle install --jobs 4 --retry 3
         bundle exec rake test