dbusServices: Allow to persist services via environment

Auto-shutdown prevents debugging a service with tools like
d-feet, so allow turning it off with an environment variable.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2169>

js/dbusServices/dbusService.js

@@ -90,6 +90,9 @@ var ServiceImplementation = class {
         if (!this._autoShutdown)
             return;
 
+        if (GLib.getenv('SHELL_DBUS_PERSIST'))
+            return;
+
         if (this._holdCount > 0)
             return;