live_build: tweak mock config used for Rawhide
    
    The numbered config file won't always exist right after branch
    (there is no fedora-40-x86_64 now, for e.g.) But the named one
    always does. This additional variable is a small price to pay for
    making the test more robust.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>