gtk2: move gtk-update-icon-cache to gtk2.out

... to be useful for regeneration when building nixos environments.
Fixes #18536 (hopefully).

nixos/modules/services/x11/desktop-managers/xfce.nix

@@ -69,7 +69,7 @@ in
     services.xserver.updateDbusEnvironment = true;
 
     environment.systemPackages =
-      [ pkgs.gtk # To get GTK+'s themes.
+      [ pkgs.gtk.out # To get GTK+'s themes and gtk-update-icon-cache
         pkgs.hicolor_icon_theme
         pkgs.tango-icon-theme
         pkgs.shared_mime_info

pkgs/data/icons/tango-icon-theme/default.nix

@@ -16,7 +16,7 @@ stdenv.mkDerivation rec {
 
   configureFlags = "--enable-png-creation";
 
-  postInstall = '''${gtk.dev}/bin/gtk-update-icon-cache' "$out/share/icons/Tango" '';
+  postInstall = '''${gtk.out}/bin/gtk-update-icon-cache' "$out/share/icons/Tango" '';
 
   meta = {
     description = "A basic set of icons";

pkgs/development/libraries/gtk+/2.x.nix

@@ -44,6 +44,8 @@ stdenv.mkDerivation rec {
 
   postInstall = ''
     moveToOutput share/gtk-2.0/demo "$devdoc"
+    # The updater is needed for nixos env and it's tiny.
+    moveToOutput bin/gtk-update-icon-cache "$out"
   '';
 
   passthru = {