(ci): replace archived actions-rs actions

[krisztianfekete]

actions-rs/toolchain and actions-rs/cargo are both archived and still on node12, so dependabot keeps opening PRs to bump it, see #318.

This PR replaces them with dtolnay/rust-toolchain, which is already pinned at the same SHA in the cross-compile job, so no new dependency.
All six cargo commands are unchanged, profile: minimal/override: true are what dtolnay does by default, and rustup component add folds into components:.

Also dropped the protoc install from the Rustfmt job, since cargo fmt never builds anything.

Job names are untouched so the checks report exactly as before. Only downside: we losethe inline error annotations actions-rs/cargo gave us, but errors are still in the logs.

Closes #318.

[SuperQ]

The GitHub Actions docs seem to indicate we could use rustup directly.

But this seems fine.

[krisztianfekete]

The GitHub Actions docs seem to indicate we could use rustup directly.

But this seems fine.

Yeah, good point, that would work too. Went with dtolnay/rust-toolchain here mainly because it was already pinned in the cross-compile job, so it's not new surface, and we can avoid the extra rustup component add lines. We can consider migrating later, though.

Thanks for the review!