Removed duplicated dag_run join in Dag.get_task_instances()

[hubert-pietron]

Related: #20248
Calling the Dag.get_task_instances() method would create warning sqlalchemy.exc.SAWarning: Pathed join target TaskInstance.dag_run has already been joined to; skipping, removed one of duplicated joins.

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[boring-cyborg]

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[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[uranusjr]

I wish SQLAlchemy should be smarter about this and automatically ignore the duplicated join; this makes this sort of query-agumenting more robust (because removing duplicated join means the outer order_by() may break in the future if the implementation of _get_task_instances() changes). But this is what it is, we all know SQLAlchemy is far from perfect.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!