pin rust toolchain version in rust-toolchain file

[the-mikedavis]

This is a less verbose alternative to #2512 that uses the rustup rust-toolchain file to pin the version of rust so new releases can't cause CI failures. Ideally actions-rs/toolchain should be able to support rust-toolchain.toml which already exists in this repo but it looks like there are a few PRs that haven't been updated in a while for adding support there. We may want to fork that action under helix-editor and merge it ourselves to make this less hacky.

[archseer]

There's already a rust-toolchain.toml which is now preferred over .rust-toolchain
https://github.com/helix-editor/helix/blob/master/rust-toolchain.toml

[the-mikedavis]

I did initially try that but it looks like the rustup version included in the actions runner is too old and it doesn't know about the .toml one, or it's a bug within actions-rs/toolchain: https://github.com/the-mikedavis/helix/runs/6516161434?check_suite_focus=true. It might be workable if we pull in actions-rs/toolchain#209 though

[the-mikedavis]

I'll try out setting up a fork of actions-rs/toolchain. Having both versions of the rust-toolchain file seems hacky