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.

mkosi.images/system/mkosi.conf

@@ -15,6 +15,9 @@ ExtraTrees=
 
 PostInstallationScripts=mkosi.sanitizers.chroot
 
+Environment=
+        SYSTEMD_REPART_OVERRIDE_FSTYPE_ROOT=%F
+
 InitrdPackages=
         btrfs-progs
         findutils

mkosi.images/system/mkosi.conf.d/10-centos/mkosi.conf

@@ -5,9 +5,14 @@ Distribution=centos
 
 [Content]
 Environment=
-        # The kernel versions in CentOS Stream 9 doesn't support orphan_file, but later versions of
-        # mkfs.ext4 enabled it by default, so we disable it explicitly.
-        Environment=SYSTEMD_REPART_MKFS_OPTIONS_EXT4="-O ^orphan_file"
+        Environment=
+                # We'd prefer to use XFS here but it fails to mount on duplicate filesystem UUIDs which
+                # happens when running tests in parallel so we use ext4 instead.
+                SYSTEMD_REPART_OVERRIDE_FSTYPE_ROOT=ext4
+                # The kernel versions in CentOS Stream 9 doesn't support orphan_file, but later versions of
+                # mkfs.ext4 enabled it by default, so we disable it explicitly.
+                SYSTEMD_REPART_MKFS_OPTIONS_EXT4="-O ^orphan_file"
+
         GIT_URL=https://git.centos.org/rpms/systemd.git
         GIT_BRANCH=c9s-sig-hyperscale
         GIT_COMMIT=8cf2aed0181920611421384f7374720db269d6c7

mkosi.images/system/mkosi.repart/10-root.conf

@@ -2,7 +2,6 @@
 
 [Partition]
 Type=root
-Format=btrfs
 CopyFiles=/
 SizeMinBytes=8G
 SizeMaxBytes=8G