Switch to `rust-toolchain` instead of `actions-rs`

[Jake-Shadle]

actions-rs has basically been dead for a while and while it works, we should switch to something maintained, and even better, simpler, such as https://github.com/dtolnay/rust-toolchain.

[repi]

Makes sense, that seems to be the one most are moving to. Don't think our action usages are that advanced either in different repos, but should try and update a few projects and see how well it works.