Use old uglifyjs on RHEL 8
    
    RHEL 8 buildroot does not have python3-rjsmin yet. Fall back to
    uglifyjs.
    
    See: https://pagure.io/freeipa/issue/8300
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>