make-fonts-cache: remove CACHEDIR.TAG file from Nix store

A CACHEDIR.TAG file indicates that the contents can be automatically
re-generated. This is not really true for Nix store paths. (Well _Nix_
can recreate them, but that's different.)

I noticed this issue as I was restoring full system backup that "for
some reason" always missed /nix/store/*-fc-cache (found by `nix-store
--verify --repair`). Turns out I was excluding caches from my backup...

pkgs/development/libraries/fontconfig/make-fonts-cache.nix

@@ -24,4 +24,8 @@ runCommand "fc-cache"
 
     mkdir -p $out
     fc-cache -sv
+
+    # This is not a cache dir in the normal sense -- it won't be automatically
+    # recreated.
+    rm "$out/CACHEDIR.TAG"
   ''