determine_rhel_state: Handle z-stream branches
    
    This function is called by the `check-tickets` task in the Gitlab
    pipelines, which now also run for Z-stream branches for some RHEL
    packages. We need to ensure that they are properly validating the rules
    for those Z-stream branches. Prior to this patch, they were being
    bundled into the handling for stream branches (which is to say their
    rules were being ignored).
    
    Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>