Add note about using dag_run.conf in BashOperator

[ashb]

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[kaxil]

Doc building error

==================== Error   2 ====================
 WARNING: Could not lex literal_block as "python". Highlighting skipped.

File path: /opt/airflow/docs/howto/operator/bash.rst (67)

  63 | 
  64 | Instead, you should pass this via the ``env`` kwarg and use double-quotes
  65 | inside the bash_command, as below:
  66 | 
  67 | .. code-block:: python
  68 | 
  69 |     bash_task = BashOperator(
  70 |         task_id="bash_task",
  71 |         bash_command='echo "here is the message: \\'$message\\'"',
  72 |         env={'message': '{{ dag_run.conf["message"] if dag_run else "" }}'},
==================================================

[ashb]

Hmmmm, I wonder what is going on between sphinx and pygmentize:

airflow ❯ pygmentize -l python -F raiseonerror < test-lex.py
    bash_task = BashOperator(
        task_id="bash_task",
        bash_command='echo "here is the message: \'$message\'"',
        env={'message': '{{ dag_run.conf["message"] if dag_run else "" }}'},

works fine and doesn't error.

[ashb]

(I tried with \ first, that failed, and with \\ too wondering if sphinx was treating \\ as something. That failed too)