Add `toolchain: github-default` or similar

[bal-e]

This is related to #63 but for a different use case. I want to write a CI job that builds documentation and reports broken doc links. I don't need the latest stable Rust for that, I'm happy to use whatever GitHub's providing in their runner images. However, setup-rust-toolchain is still a nice action to use because it tells GitHub how to turn warnings into annotations on source files. This is basically a small performance optimization, but allowing the toolchain input to say "whatever GitHub already has" would be perfect for me. If you're open to that, I'm happy to make a PR.