GitHub Actions: install clippy after toolchain
GitHub Actions: install clippy after toolchain

Doing it the other way around means that when we switch to Beta, Clippy
won't have been installed on Beta (even though it's already installed on
Stable)

petertseng/exercism-rust,exercism/xrust,exercism/xrust,petertseng/exercism-rust,petertseng/exercism-rust