Replace unmaintained actions-rs/toolchain action in CI workflows
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.
To get rid of those warnings the occurrences of
`actions-rs/toolchain` are replaced by `dtolnay/rust-toolchain`.
