Workaround f30 compose/buildroot mismatch
    
    The f30 tag is currently a lot fresher than what's in the repos,
    because we haven't had a compose for five days. This causes
    problems in some F30 update tests because the update works fine
    with what's in the `f30` tag but not what's on the mirrors. To
    deal with this until we get a successful compose, let's just
    enable the f30-build repo in the update test process...
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>