makeFontsCache: fix cross compilation

lifting the `callPackage` to scope level ensures splicing works as expected.
we still need to emulate fc-cache to produce usable cache entries though.

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

@@ -1,13 +1,20 @@
 {
-  runCommand,
-  lib,
+  buildPackages,
   fontconfig,
+  lib,
+  runCommand,
+  stdenv,
+}:
+let
+  fontconfig' = fontconfig;
+in
+{
+  fontconfig ? fontconfig',
   fontDirectories,
 }:
 
 runCommand "fc-cache"
   {
-    nativeBuildInputs = [ fontconfig.bin ];
     preferLocalBuild = true;
     allowSubstitutes = false;
     passAsFile = [ "fontDirs" ];
@@ -29,8 +36,11 @@ runCommand "fc-cache"
     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

@@ -8764,10 +8764,7 @@ with pkgs;
 
   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;