Clean up the exception handler when run_as_user is the airflow user

[squingo44]

During the heartbeat check, the local_task_job_runner performs a check on the job's PID and raises an exception if it doesn't match what it expects.

In the case that the job is a child process (ie. when running as another user), this check will instead occur on the job's parent PID.

The condition for triggering this alternate route is run_as_user having a non-None value. This condition is correct, except for the case where run_as_user has been set to user as which Airflow is already running, in which case run_as_user is ignored when the TaskRunner is initialialized.

This means that whenever an Airflow user sets run_as_user to the user as which they're already running Airflow, they would get an error, and be told to unset the run_as_user attribute. However, there is a legitimate use case for this: run_as_user can be used to override default_impersonation back to the airflow_uid user.

This change updates the "is_child_process" condition within the exception handler to reflect this use case. It makes this use case succeed silently instead of throwing an error.

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[potiuk]

Nice catch. Can you add a test case for that ?