Provide a built-in way for rustup to adapt to a CI environment

Discussed at the rust all-hands, it was suggested that it'd be good if rustup had better support for CI scenarios so it's less necessary for people to rely on third parties to provide them. This issue is intended to explore the existing options and see what we can adopt into rustup itself and how we can do so.

The most current github actions for rustup are rust-toolchain and setup-rust-toolchain.

What the actions do

• sets environment variables by default such as:
  • CARGO_UNSTABLE_SPARSE_REGISTRY=true
  • CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse (or =git for older cargo)
  • CARGO_INCREMENTAL=0
  • CARGO_PROFILE_DEV_DEBUG=0
  • RUST_BACKTRACE=short
  • CARGO_TERM_COLOR=always
• runs rustup toolchain install to install the current default toolchain
• Uses --profile minimal and --no-self-update when installing specified toolchains
• provides problem matchers for cargo and rustfmt issues

They can also provide caching but that is deemed out of scope for rustup.