nixos/gdm: Fix missing icons

Recent `wrapGAppsHook` change stops `adwaita-icon-theme` from being added to `XDG_DATA_DIRS`:
b1e73fa2e0
Since `display-manager.service` does not have `/run/current-system/sw/share` in `XDG_DATA_DIRS`, it does not pick up the globally installed icon theme either, preventing icons from showing.

Let’s make Adwaita available to fix that for now.

Fixes: https://github.com/NixOS/nixpkgs/issues/171692

nixos/modules/services/x11/display-managers/gdm.nix

@@ -140,8 +140,13 @@ in
         environment = {
           GDM_X_SERVER_EXTRA_ARGS = toString
             (filter (arg: arg != "-terminate") cfg.xserverArgs);
-          # GDM is needed for gnome-login.session
-          XDG_DATA_DIRS = "${gdm}/share:${cfg.sessionData.desktops}/share:${pkgs.gnome.gnome-control-center}/share";
+          XDG_DATA_DIRS = lib.makeSearchPath "share" [
+            gdm # for gnome-login.session
+            cfg.sessionData.desktops
+            pkgs.gnome.gnome-control-center # for accessibility icon
+            pkgs.gnome.adwaita-icon-theme
+            pkgs.hicolor-icon-theme # empty icon theme as a base
+          ];
         } // optionalAttrs (xSessionWrapper != null) {
           # Make GDM use this wrapper before running the session, which runs the
           # configured setupCommands. This relies on a patched GDM which supports