fontconfig: document some limitations of the cache

hosts/common/programs/fontconfig.nix

@@ -14,6 +14,10 @@ let
   # and that needs to either be added to the sandbox of *every* app,
   # or font-heavy apps are several *seconds* slower to launch.
   #
+  # N.B.: fc-cache produces output based on the architecture it's run on,
+  # e.g. /var/fontconfig/cache/<hash>-aarch64.cache-9 or <hash>-x86_64.cache-9
+  # this means we'd better emulate it -- don't run the host binary.
+  #
   # TODO: upstream this into `make-fonts-cache.nix`?
   cache = (pkgs.makeFontsCache { fontDirectories = config.fonts.packages; }).overrideAttrs (upstream: {
     buildCommand = lib.replaceStrings