fix GCSToGCSOperator bug when copy single object with replace to False (

#45181)

providers/src/airflow/providers/google/cloud/transfers/gcs_to_gcs.py

@@ -304,7 +304,7 @@ def _ignore_existing_files(self, hook, prefix, **kwargs):
                 dest_object.replace(self.destination_object, prefix, 1) for dest_object in destination_objects
             ]
 
-        objects = set(objects) - set(existing_objects)
+        objects = list(set(objects) - set(existing_objects))
         if objects:
             self.log.info("%s files are going to be synced: %s.", len(objects), objects)
         else: