Fix PackedScene::get_last_modified_time() always returns 0

The variables operated by `PackedScene::set_last_modified_time()`
and `PackedScene::get_last_modified_time()` are different.

(cherry picked from commit 22edef14c3)

scene/resources/packed_scene.h

@@ -255,6 +255,7 @@ public:
 	virtual void set_path(const String &p_path, bool p_take_over = false) override;
 #ifdef TOOLS_ENABLED
 	virtual void set_last_modified_time(uint64_t p_time) override {
+		Resource::set_last_modified_time(p_time);
 		state->set_last_modified_time(p_time);
 	}