CI: determine rust channel from `rust-toolchain.toml`

[the-mikedavis]

The actions-rs/toolchain action used in CI can determine which rust channel to use from the rust-toolchain.toml file rather than the explicit toolchain: stable argument passed currently.

It doesn't work perfectly at the moment. There's an upstream PR that will fix the functionality: actions-rs/toolchain#209.

For now we can fork the toolchain action and merge that PR. Eventually we should switch back to the upstream when that PR is merged.

See also:

• pin rust toolchain version in rust-toolchain file #2513
• Discussion of lints CI in Fix panic when reloading a shrunk file #2506

[kpbaks]

Is this issue still relevant?
We no longer use actions-rs/toolchain but instead dtolnay/rust-toolchain. This action does not support, and does not intend to support, parsing rust-toolchain.toml for getting the Rust channel.

[the-mikedavis]

It would be nice to have the CI pick up the toolchain automatically from the rust-toolchain.toml file but the current stuff we do with

helix/.github/workflows/build.yml

Lines 11 to 12 in 56fa9bf

	env:
	MSRV: "1.82"

is sufficient, it's not too much of a pain to keep that in line and all of the MSRV stuff is documented in https://github.com/helix-editor/helix/blob/56fa9bf7c1652cd9239d9669d3d70190889f1ea9/docs/CONTRIBUTING.md#minimum-stable-rust-version-msrv-policy

So yeah I agree it's fine to close this out