mkosi: Use the "default" root filesystem for each distribution

mkosi: Use the "default" root filesystem for each distribution

Let's not insist on btrfs everywhere. 93440db8b5eae1244aaf5fecfa68050a8b26f3e3
switched us back to btrfs as we wanted to rely on the fact it records
timestamps properly. Since we now prefer to do incremental builds on the host
with "mkosi -t none" we don't mind anymore that timestamps are not recorded
properly so we're not forced to use btrfs anymore.

This also increases test coverage as we'll now test with different root
filesystems.