mate: Install mate-panel-with-applets by default

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

···
       pkgs.mate.mate-session-manager
     ];
 
-    # Let mate-panel find applets
-    environment.sessionVariables."MATE_PANEL_APPLETS_DIR" = "${config.system.path}/share/mate-panel/applets";
-    environment.sessionVariables."MATE_PANEL_EXTRA_MODULES" = "${config.system.path}/lib/mate-panel/applets";
-
     # Debugging
     environment.sessionVariables.MATE_SESSION_DEBUG = mkIf cfg.debug "1";
 

pkgs/desktops/mate/default.nix

···
       mate-icon-theme
       mate-menus
       mate-notification-daemon
-      mate-panel
+      mate-panel-with-applets
       mate-polkit
       mate-session-manager
       mate-settings-daemon