Update CI

[qryxip]

• Bumps actions/checkout to v3.

• Stops using actions-rs.

  actions-rs has been unmaintained for almost three years, and is going to stop working. https://github.com/actions-rs/toolchain/issues/216

  Instead of actions-rs/toolchain, uses dtolnay/rust-toolchain. It is used in his own huge number of repositories, such as syn, anyhow, and cxx.