copr-be: don't attempt to install mock from koji
    
    We don't need it ATM, and currently it often fails for some reason
    (perhaps related to Fedora outages):
    
    TASK [install the latest mock and mock-core-configs from updates-testing] ******
    Friday 19 June 2020  05:54:19 +0000 (0:01:07.155)       0:07:47.913 ***********
    fatal: [52.90.206.190]: FAILED! => {"changed": false, "msg": "Failure
    downloading
    https://kojipkgs.fedoraproject.org/packages/mock/2.2/1.fc31/noarch/mock-2.2-1.fc31.noarch.rpm,
    Connection failure: The read operation timed out", "results": []}