pantheon.elementary-greeter: pick up hardcoded icon theme

wrapGAppsHook no longer pick up XDG_ICON_DIRS for us.

pkgs/desktops/pantheon/desktop/elementary-greeter/default.nix

@@ -91,8 +91,11 @@ stdenv.mkDerivation rec {
       # for the compositor
       --prefix PATH : "$out/bin"
 
-      # the theme is hardcoded
+      # the GTK theme is hardcoded
       --prefix XDG_DATA_DIRS : "${elementary-gtk-theme}/share"
+
+      # the icon theme is hardcoded
+      --prefix XDG_DATA_DIRS : "$XDG_ICON_DIRS"
     )
   '';