pass along the backfill command pool to the ti

Author: kimyen

airflow/jobs/backfill_job.py

@@ -535,6 +535,9 @@ def _per_task_process(key, ti, session=None):  # pylint: disable=too-many-return
                         ):
                             cfg_path = tmp_configuration_copy()
 
+                        # pass along the backfill command pool arg to the ti
+                        if self.pool:
+                            ti.pool = self.pool
                         executor.queue_task_instance(
                             ti,
                             mark_success=self.mark_success,