Replace actions-rs/toolchain action

[JohnTitor]

On CI we use actions-rs/toolchain but it's currently unmaintained and has some issues like actions-rs/toolchain#220.
An alternative in my mind is https://github.com/dtolnay/rust-toolchain or using toolchains on GHA directly.
Does someone have any objections? If not, I'm going to replace them on the repos under the org.

[cathay4t]

I noticed alacritty is using rustup directly. Maybe we should the same.