mtime isn't reliable to detect updates of fonts on OSTree based system
since they reset mtime to 0 for system files.
Due to this, there are the situation likely to happen where mtime is
newer but content is older.

Fortunately, OSTree based system requires reboot to deploy changes. so
we can assume we won't see any changes on system fonts. so system caches
are always up-to-date. we can ignore meta data for system fonts in
user caches.