Use GitHub runner built-in Rust support

[jobarr-amzn]

actions-rs/toolchain is unmaintained, see: actions-rs/toolchain#216

The GitHub docs recommend using the built-in rustup available in GitHub runners: https://docs.github.com/en/actions/tutorials/build-and-test-code/rust

Perhaps we want to use dtolnay/rust-toolchain instead?

We may want to cache dependencies? See:

• https://docs.github.com/en/actions/tutorials/build-and-test-code/rust#caching-dependencies
• Swatinem/rust-cache

This PR is meant to at least start a conversation about what we want to do with CI builds going forward.

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