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