fontconfig cache is not reproducible in Buster
See attached diffoscope output.
I don’t remember if we expected Buster to have everything we need, or if there’s still some upstream or Debian bug report open about that. I guess that’s https://bugs.debian.org/864082, which is not marked as forwarded upstream (strange, probably a mere oversight). We could of course build a patched fontconfig package as we do for Stretch, but it would be nice to seize this opportunity to try harder to upstream this.