koji-rawhide: set skip_if_unavailable=1 for now due to 404 issues
    
    We're seeing a lot of tests fail on 404s when trying to access
    the koji-rawhide repo (the repo for the Rawhide build tag, which
    we use to get packages tagged since the last compose. nirik is
    trying to figure this out from the server end, but for now at
    least, let's mark the repo as skip_if_unavailable. This should
    mean that if we hit a 404, the test will continue, it just won't
    have access to the packages from that repo. Occasionally this
    will cause a problem - a false failure or false pass - but this
    still seems better than every test that hits it failing. The
    false pass case is the most concerning, but I would hope in that
    case some other tests from the same update would fail, making it
    not an issue.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>