Bump installer image rootfs size on Fedora 30 update tests
    
    https://bodhi.fedoraproject.org/updates/FEDORA-2020-1070052d10#comment-1284223
    It seems the rootfs on the Fedora 30 installer images we build at
    present has gotten very big, so big that an update which contains
    some very slightly larger firmware packages causes the rootfs to
    be completely full (though lorax doesn't fail) and the image
    doesn't boot.
    
    I don't yet know when or why the rootfs got that big, but it's
    not really a bug in this update, so for now let's just tell
    lorax to use a bigger rootfs so the tests pass for this and any
    similar future updates, until I can maybe find time to pinpoint
    the culprit more precisely.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>