how to pin `dtolnay/rust-toolchain`

dtolnay/rust-toolchain does not use tags: the commonly-used `stable` but also versions like `1.89.0` are not tags, but branches. https://github.com/dtolnay/rust-toolchain#choice-of-full-length-commit-sha recommends picking commits that are part of the master branch, warning other commits will be garbage-collected - so presumably neither the 'channel branches' nor the 'version branches' are expected to be usable longer-term (?). Perhaps we should track the latest commits to `master` of the action? (the action has a parameter to select which toolchain to install, so that might work fine).

Alternatively, perhaps we could recommend another method of installing a Rust toolchain? Perhaps https://github.com/actions-rust-lang/setup-rust-toolchain ?

/cc @snazy @potiuk @kevinjqliu

Search the repository for references to dtolnay/rust-toolchain and review its upstream choice-of-full-length-commit-sha guidance. Compare that approach with actions-rust/setup-rust-toolchain, then document or implement the selected long-term toolchain pinning method. Done means the repository has one clear, maintainable recommendation.

Written by the indexing model from the issue text.