copr: builders: better tweak mock-core-configs
    
    We first run the copr-update-builder script, so it does it's work.
    Then we can try to update to our own (newer only!) version of
    mock-core-configs, and re-run the copr-update-builder.
    
    We though have to stop removing the old .rpmnew files blindly, this
    needs patch against copr/copr.git.