Use a second disk for the update repo in image build tests
    
    This tweaks the update live and installer image build tests to
    store the update repo on a separate disk. This should hopefully
    avoid the tests failing due to insufficient disk space when the
    update is huge (e.g. KDE or GNOME megaupdates).
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>