mkosi: Switch back to btrfs

The next commit will introduce a way to iterate on integration
tests which depends on btrfs specific features.

We leave CentOS on ext4 as its kernel does not support btrfs.

mkosi.conf

@@ -46,9 +46,6 @@ ExtraTrees=
 
 Initrds=%O/initrd
 
-Environment=
-        SYSTEMD_REPART_OVERRIDE_FSTYPE_ROOT=%F
-
 # Disable relabeling by default as it only matters for TEST-06-SELINUX, takes a non-trivial amount of time
 # and results in lots of errors when building images as a regular user.
 SELinuxRelabel=no

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

@@ -8,8 +8,9 @@ Release=10
 
 [Content]
 Environment=
-        # We'd prefer to use XFS here but it fails to mount on duplicate filesystem UUIDs which
+        # CentOS doesn't support btrfs so we have to use something else. We'd prefer to use XFS here but it
-        # happens when running tests in parallel so we use ext4 instead.
+        # 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.

mkosi.repart/10-root.conf

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