[Fedora-spins] Nightly composes using XZ spins

The nightly composes switched over to using XZ today. I am not sure if something else changed in rawhide last night, but the kde and desktop spins each dropped about 130 MB which is a lot better than I expected (which was mostly based on the effect I had seen on the games spin in tests). If you see any problems that look like they might be related using XZ for the spins please file a bug against livecd-tools.