Upcoming Ubuntu 24.04 runner may not have Clippy pre-installed

[laniakea64]

In https://github.com/NoahTheDuke/vim-just/issues/78 / https://github.com/NoahTheDuke/vim-just/commit/53459ce5cc358ad3844f3ee5ac57ec3e4e3097b5 we removed the "Install Rust Toolchain Components" step, because it's redundant in the current CI runner image. But according to documentation of upcoming Ubuntu 24.04 runner and https://github.com/actions/runner-images/issues/9848 , looks like Clippy will no longer be pre-installed in our CI when that runner image becomes ubuntu-latest. Which https://github.com/actions/runner-images/issues/9848#issuecomment-2111684278 suggests will likely happen sometime next month.

We need to either reinstate use of rustup for Clippy or temporarily pin our CI to ubuntu-22.04.