pantheon.wingpanel: pick up required icon theme

Looks like the theme is not hardcoded for wingpanel, but wingpanel is called
by greeter which hardcodes the themes. I decide I will just do something
safe before I am confident enough and know what is happening.

pkgs/desktops/pantheon/desktop/wingpanel/default.nix

@@ -64,8 +64,11 @@ stdenv.mkDerivation rec {
 
   preFixup = ''
     gappsWrapperArgs+=(
-      # this theme is required
+      # this GTK theme is required
       --prefix XDG_DATA_DIRS : "${elementary-gtk-theme}/share"
+
+      # the icon theme is required
+      --prefix XDG_DATA_DIRS : "$XDG_ICON_DIRS"
     )
   '';