Replace unmaintained actions-rs/toolchain action in CI workflows

[striezel]

Basically all of the actions-rs/* actions are unmaintained. See https://github.com/actions-rs/toolchain/issues/216 for more information. Due to their age they generate several warnings in CI runs, for example in https://github.com/RustCrypto/hashes/actions/runs/3716377913:

Node.js 12 actions are deprecated. For more information see: https://github.blog/changelog/2022-09-22-github-actions-all-actions-will-begin-running-on-node16-instead-of-node12/. Please update the following actions to use Node.js 16: actions-rs/toolchain@v1

To get rid of some of those warnings the occurrences of actions-rs/toolchain are replaced by dtolnay/rust-toolchain.

[tarcieri]

Thanks. We've been making this change elsewhere (e.g. https://github.com/RustCrypto/formats/pull/887) but there are quite a few repos to update

[tarcieri]

Hmm, I think GitHub Actions is having some issues... jobs are showing up as cancelled but without failures

[tarcieri]

@striezel can you commit --amend and try re-pushing? CI seems wedged

[striezel]

@striezel can you commit --amend and try re-pushing? CI seems wedged

Done and force pushed. And while I was at it, I also replaced the remaining target: ... with targets: ... for consistency.

[newpavlov]

Thank you!