Fix cache conflicts on OSTree based system

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.
3 jobs for sb-timestamp in 6 minutes and 28 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Build
passed #2646500
mingw-build

00:05:29

passed #2646496
shared-build

00:06:28

passed #2646498
static-build

00:04:50