nixos/fonts: ship fontconfig caches even for cross-compiled hosts (#391305)

nixos/modules/config/fonts/fontconfig.nix

···
       <fontconfig>
         <!-- Font directories -->
         ${lib.concatStringsSep "\n" (map (font: "<dir>${font}</dir>") config.fonts.packages)}
-        ${lib.optionalString (pkgs.stdenv.hostPlatform == pkgs.stdenv.buildPlatform) ''
+        ${lib.optionalString (pkgs.stdenv.hostPlatform.emulatorAvailable pkgs.buildPackages) ''
           <!-- Pre-generated font caches -->
           <cachedir>${cache}</cachedir>
           ${lib.optionalString (pkgs.stdenv.hostPlatform.isx86_64 && cfg.cache32Bit) ''

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

···
 {
+  buildPackages,
+  fontconfig,
+  lib,
   runCommand,
-  lib,
-  fontconfig,
+  stdenv,
+}:
+let
+  fontconfig' = fontconfig;
+in
+{
+  fontconfig ? fontconfig',
   fontDirectories,
 }:
 
 runCommand "fc-cache"
   {
-    nativeBuildInputs = [ fontconfig.bin ];
     preferLocalBuild = true;
     allowSubstitutes = false;
     passAsFile = [ "fontDirs" ];
···
     cat "$fontDirsPath" >> fonts.conf
     echo "</fontconfig>" >> fonts.conf
 
+    # N.B.: fc-cache keys its cache entries by architecture.
+    # We must invoke the host `fc-cache` (not the build fontconfig) if we want
+    # the cache to be usable by the host.
     mkdir -p $out
-    fc-cache -sv
+    ${stdenv.hostPlatform.emulator buildPackages} ${lib.getExe' fontconfig "fc-cache"} -sv
 
     # This is not a cache dir in the normal sense -- it won't be automatically
     # recreated.

pkgs/top-level/all-packages.nix

···
 
   makeFontsConf = callPackage ../development/libraries/fontconfig/make-fonts-conf.nix { };
 
-  makeFontsCache = let fontconfig_ = fontconfig; in {fontconfig ? fontconfig_, fontDirectories}:
-    callPackage ../development/libraries/fontconfig/make-fonts-cache.nix {
-      inherit fontconfig fontDirectories;
-    };
+  makeFontsCache = callPackage ../development/libraries/fontconfig/make-fonts-cache.nix { };
 
   freenect = callPackage ../development/libraries/freenect {
     inherit (darwin.apple_sdk.frameworks) Cocoa GLUT;