standard-test-source: install rpm-build only as root
    
    Without this patch, people running STR against a remote machine
    without root cannot use this role ...
    
    NOTE: This patch is an exception to STI by the OSCI team. All fixes
    like this need to be approved for inclusion, as they are not compliant
    to STI.
    
    Signed-off-by: Miroslav Vadkerti <mvadkert@redhat.com>