Make pre-release check a soft failure on osbuild images
    
    osbuild is struggling with the pre-release warnings ATM:
    
    https://pagure.io/fedora-iot/issue/57
    https://github.com/osbuild/images/issues/515
    
    Until that mess is cleaned up we can't really make sensible
    assertions for osbuild images, so let's make the check a soft
    failure for now (now we know about the bugs, we want to let
    the rest of the tests run and not block them on this).
    
    Note for IoT the behaviour has never really been correct (IoT
    images never get pre-release warnings), but the logic in this
    check matches the wrong behaviour (IoT composes always have
    RC- labels even when they're clearly not RCs), so the test
    didn't fail. While fixing this in osbuild we might try to get
    'correct' behaviour for IoT, and then we'd need to tweak the
    logic here.
    
    While we're at it, tweak the implementation a bit; without this
    tweak, implementing this 'soft fail if osbuild' behaviour is more
    awkward and ugly.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>