install-rust

A small github action to install rustup and a Rust toolchain. This is generally expressed inline, but it was repeated enough in this repository it seemed worthwhile to extract.

Some gotchas:

Can't --self-update on Windows due to permission errors (a bug in Github Actions)
rustup isn't installed on macOS (a bug in Github Actions)

When the above are fixed we should delete this action and just use this inline:

- run: rustup update $toolchain && rustup default $toolchain
  shell: bash