koji-wrapper: Run koji runroot with fresh credentials cache
    
    If the koji profile we are using is configured to use keytab, we should
    run koji executable with a fresh credentials cache. Otherwise we risk a
    race condition as multiple processes will trample over the same
    directory in /tmp/krbcc_0.
    
    This is currently only implemented for calling `koji runroot`. We might
    need to do it for other commands as well (currently there is a sleep to
    avoid the race condition for other commands).
    
    Fixes: https://pagure.io/releng/issue/6715
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>