Rollup merge of #77016 - Mark-Simulacrum:clippy-tests, r=pietroalbini

Test clippy on PR CI on changes

This runs the tools builder (which builds and tests tools, including clippy) when the clippy submodule changes. This essentially returns us to the prior state when clippy was a submodule; it makes sense for us to test it on CI when it changes. It might make sense for it to be tested regardless of changing but it is somewhat rare for it to fail and we don't want to add to CI time for the majority of PRs which don't affect it.

Fixes #76999.