ResourceLoader: Fixup resource changed feature

This is a complement to: https://github.com/godotengine/godot/pull/96593

(cherry picked from commit 97197ff5e9c73ffbb2e3822d40a63bc3f8c47373)

core/io/resource_loader.cpp

@@ -826,6 +826,8 @@ Ref<Resource> ResourceLoader::_load_complete_inner(LoadToken &p_load_token, Erro
 		load_task_ptr = &load_task;
 	}
 
+	thread_load_mutex.unlock();
+
 	Ref<Resource> resource = load_task_ptr->resource;
 	if (r_error) {
 		*r_error = load_task_ptr->error;
@@ -863,6 +865,8 @@ Ref<Resource> ResourceLoader::_load_complete_inner(LoadToken &p_load_token, Erro
 		}
 	}
 
+	thread_load_mutex.lock();
+
 	return resource;
 }