Run CI on `ubuntu-latest` again

[xFrednet]

Bors CI is currently broken on ubuntu-latest due to a missing dependency. #10039 moved the CI to ubuntu-20.04 in the meantime. This is a reminder issue, to move the CI back, once the dependency has been released.

See: https://rust-lang.zulipchat.com/#narrow/stream/257328-clippy/topic/Broken.20CI.20gitlib2.20is.20missing/near/314205535