pass along the backfill command pool to the task instance

[kimyen]

To be used together with adding new pool for backfill in airflow-sources and prevent task instance from being assigned to a pool by its operator.

[whitleykeith]

@kimyen I looked into where the task instance get's assigned the pool in airlfow-sources, and I think we can check the dag_run_id of the task and assign it there so we don't need to touch our fork

https://github.com/github/airflow-sources/blob/master/airflow_local_settings.py#L53 <-- This is where we override the pool for the task instance.

We can change the logic in the called function to inspect the dag run id.