boot/ostree-remount.service: Run before plymouth-read-write.service

The plymouth service needs a writable /var, so ensure that we run
before it does.

src/boot/ostree-remount.service

@@ -23,7 +23,9 @@ OnFailure=emergency.service
 Conflicts=umount.target
 After=-.mount
 After=systemd-remount-fs.service
-Before=local-fs.target umount.target systemd-random-seed.service
+Before=local-fs.target umount.target
+# Other early boot units that need to write to /var
+Before=systemd-random-seed.service plymouth-read-write.service
 
 [Service]
 Type=oneshot