[perf] Write UTC dates into performance logs

Instead of writing a local date without a timezone, write a UTC
date with a trailing Z.

src/gnome-shell.in

@@ -439,7 +439,7 @@ def run_performance_test():
         # version here for simplicity. Using json.dump(indent=0) doesn't real
         # improve the readability of the output much.
         report = {
-            'date': datetime.datetime.now().isoformat(),
+            'date': datetime.datetime.utcnow().isoformat() + 'Z',
             'events': events,
             'monitors': monitors,
             'metrics': metric_summaries,