Kill external for checks

We ran into the issue again that the format CI job reported the submodule as changed and thus failed (see !99 (merged))

I think this happens when a build job checks out the submodules, and then one of the checker jobs runs on the same machine with the same build dir with git submodule strategy disabled. It leaves the submodules as is and continues.

This MR explicitly kills the external/* submodules before executing. This should hopefully fix this once and for all.