append to CMAKE_PREFIX_PATH instead of setting executorch_DIR

[swolchok]

I think the _DIR mechanism is too low-level -- it helps find , but not any dependencies that also installed next to itself. CMAKE_PREFIX_PATH does both.

[swolchok]

Stack from ghstack (oldest at bottom):

• Use CMake EXPORT #8954
• -> append to CMAKE_PREFIX_PATH instead of setting executorch_DIR #12618
• Turn BUILD_TESTING off if EXECUTORCH_BUILD_TESTS is off #12566

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12618

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❌ 3 New Failures

As of commit f5bcdcc with merge base 00b02f1 ():

NEW FAILURES - The following jobs have failed:

• pull / test-eval_llama-mmlu-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 51f01a0b3b4ddee08dcc52818472a99be34f69f055dc60c7ce1b57c6470c34c5 /exec failed with exit code 1
• pull / test-llama-runner-linux-android / linux-job (gh)
  RuntimeError: Command docker exec -t ea178a0a907034cac8e3a516498184281ecd80c94fc1609b91f594f6f9f9270d /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  backends/xnnpack/test/ops/test_to_copy.py::TestChannelsLastTaggedReshapePass::test_quantized_to_copy

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