Add a test to ensure that PT built from the pinned commit is used

[huydhn]

This is to prevent regression like #6564. If nightly is accidentally used on CI instead of PT built from the pinned commit at ci_commit_pins/pytorch.txt. The check will fail.

Because setup-linux script is used on both ET and PT CI, the latter will also have this check in place after the next ET pin commit update.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/6785

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❌ 1 New Failure

As of commit befe2d2 with merge base c8967da ():

NEW FAILURE - The following job has failed:

• pull / unittest / linux / linux-job (gh)
  extension/llm/modules/test/test_mha.py::AttentionTest::test_attention_executorch

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