ci: Terminate capture-devcoredump before tarring up artifacts.

Every few weeks we'd get a flake where the script noticed the devcore right as we were wrapping up, and then tar would exit because the file it was tarring changed underneath it. Just kill devcores before we do that -- even if we kill it while it's working, it's so rare that it probably won't bother anyone.

Fixes: #5867 (closed)