perf-tool: Only 'restore' gnome-shell if run with --replace

Otherwise the perf tool will fork and spawn a gnome-shell process
that'll just go and fail.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1349>

src/gnome-shell-perf-tool.in

@@ -320,7 +320,7 @@ if args:
 
 normal_exit = run_performance_test()
 if normal_exit:
-    if not options.hwtest:
+    if not options.hwtest and options.replace:
         restore_shell()
 else:
     sys.exit(1)