Don't expect Atomic images for recent Rawhide / Branched
We stopped building them for F30+ on 2019-02-14. That was before
F30 branched, so for Branched the condition is simple. For
Rawhide we'll just use the date as an integer, I think that
should be fine...
Signed-off-by: Adam Williamson <awilliam@redhat.com>