[red-knot] Avoid running ecosystem checks when not necessary

[MichaReiser]

Most recent example #16493 (comment)

The PR doesn't change any files outside of Red Knot. The ecosystem checks should not run.

[AlexWaygood]

Most recent example #16493 (comment)

I think an ecosystem check ran on that PR because this commit in the PR (which has now been force-pushed away) modified code in the ruff_python_ast crate: dbae500. I don't see ecosystem checks on most red-knot PRs

[AlexWaygood]

(Previous discussion in #16078)