Only build the UI with uglifyjs on RHEL 8
    
    The previous expression tested for RHEL or RHEL-like
    systems to use uglifyjs. Tighten that up to only RHEL 8
    so future RHEL can use rjsmin.
    
    https://pagure.io/freeipa/issue/8669
    
    Signed-off-by: Rob Crittenden <rcritten@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>