[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
• Set up CMake build for extension/evalue_util #12691
• Build EXTENSION_DATA_LOADER with size_test #12678
• Prefix target_link_options_shared_lib with ExecuTorch #12667
• 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

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❌ 2 New Failures

As of commit f9b96eb with merge base 67154d0 ():

NEW FAILURES - The following jobs have failed:

• pull / test-eval_llama-mmlu-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 941b68a375d60ee3cc8485a0164cb0b75efce2a865cb7b0b884cfbdd2e509c58 /exec failed with exit code 1
• trunk / test-arm-backend (test_pytest_ops_ethosu_fvp) / linux-job (gh)
  RuntimeError: Command docker exec -t a687dcf3f28fa33760f9b1e2d7039ca1d06016c78c34d40042da5507f1e9f7cf /exec failed with exit code 1

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