prci: increase gating tasks priority
    
    Sometimes the gating tasks (build and jobs) are blocked because of nightly
    regression remaining tasks are in progress. The reason is because nightly
    regressions are not finished or they are re-triggered during day-time.
    Gating tasks are blocked because they have same priority than nightly tasks.
    
    This commit increases gating tasks priority so the testing of pull requests
    will not be blocked anymore.
    
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>