extensionSystem: Fix opening Extensions app from notification

Launching the app is implemented by the source's open() method, but
only external notifications are hooked up to call into the source
when no default action was provided.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1595>

js/ui/extensionSystem.js

@@ -252,6 +252,8 @@ var ExtensionManager = class {
             let notification = new MessageTray.Notification(source,
                 _('Extension Updates Available'),
                 _('Extension updates are ready to be installed.'));
+            notification.connect('activated',
+                () => source.open());
             source.showNotification(notification);
         }
     }