Make linter action more ergonomic

[BD103]

Context

The Github Action we provide for the linter isn't great. It works well for installing from a release or from main, but breaks down when installing a specific commit. This is because we currently hardcode values in action.yml:

bevy_cli/bevy_lint/action.yml

Lines 18 to 19 in ff8d39a

	# This must be kept in sync with `rust-toolchain.toml`.
	toolchain: nightly-2025-05-14

bevy_cli/bevy_lint/action.yml

Lines 25 to 31 in ff8d39a

	# The toolchain must be kept in sync with `rust-toolchain.toml`. The `--branch main` should
	# be swapped with `--tag lint-vX.Y.Z` for releases.
	rustup run nightly-2025-05-14 cargo install \
	--git https://github.com/TheBevyFlock/bevy_cli.git \
	--branch main \
	--locked \
	bevy_lint

This is an extra maintenance burden that we shouldn't have to do! If the user wants to install a specific commit of the linter, it should work as expected instead of installing the main branch:

# Should install this specific commit of the linter, but installs `main` instead.
- name: Install `bevy_lint`
  uses: TheBevyFlock/bevy_cli/bevy_lint@ff8d39a

Solution

Thankfully, the fix is easier than expected. action.yml is a composite action, meaning it's a single YAML file that slings around other actions and shell scripts to do things. The great thing about this is that, when an action runner downloads a composite action, it downloads the entire repository. That means rust-toolchain.toml and the correct source code for bevy_lint already exist, we just have to install them!

This PR does just that. It uses the venerable Taplo to read rust-toolchain.toml and then runs cargo install --path ... to install the linter. The action especially uses the ${{ github.action_path }} variable to locate the bevy_lint folder.

Testing

linter-action.yml is doing a lot of the heavy-lifting here by testing the linter action in CI. I'll also spin up a separate repository to prove it works for others as well!

[TimJentzsch]

I cant test right now, but the code makes sense!
If it works in a separate repository feel free to merge this :)

[TimJentzsch]

Nit: We could remove the env variable here and directly use the step output, right?
No strong preference though

[BD103]

I'd prefer not to. While it's not an issue in this specific case, directly inlining templates can lead to code injection vulnerabilities, so I avoid doing so out of habit! (It's the same reason I use "${VAR}" :)

[TimJentzsch]

Ok, that's fine for me :)

[TimJentzsch]

Since we control the toolchain file and it doesn't look too complex, I wonder if we could accomplish the same without installing an additional tool.
That could speed up the CI time.

On the other hand, we'd need to be careful to make it work on all platforms, so it probably makes sense to investigate this in a separate issue.

[BD103]

I think this would be a good follow-up. We could probably use grep to capture the toolchain version with a few clever comments, like what Clippy does.

[BD103]

Tested in BD103/linter-action/test, specifically this run. Merging now!