[easy] Allow user to set ANDROID_NDK in build_llama_android script

[swolchok]

The way it was written required hand-hacking to build locally. Now it respects ANDROID_NDK from the environment.

[swolchok]

Stack from ghstack (oldest at bottom):

• Use CMake EXPORT #8954
• append to CMAKE_PREFIX_PATH instead of setting executorch_DIR #12618
• -> [easy] Allow user to set ANDROID_NDK in build_llama_android script #12662
• 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/12662

• 📄 Preview Python docs built from this PR

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❌ 1 New Failure

As of commit 0eb3d8d with merge base 4d7f9ca ():

NEW FAILURE - The following job has failed:

• pull / test-eval_llama-mmlu-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 3a53b9dc32725ce7844aeedb851ac9c81d458b7efae7248a3c594a2b63f0e875 /exec failed with exit code 1

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