Define PYTHON_EXECUTABLE only once in cmake

[jathu]

Summary

There is no need to redefine this over and over again. Do it once at the root. Lmk if I'm missing something:

$ rg -t cmake -g '!examples' "resolve_python"
$ rg -t cmake -g '!examples' "NOT PYTHON_EXECUTABLE"
$ rg -t cmake -g '!examples' "python3"
$ rg -t cmake -g '!examples' "python\s"

Test plan

CI

cc @larryliu0820

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/10911

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❌ 1 Cancelled Job, 1 Pending, 1 Unrelated Failure

As of commit b0a2cc6 with merge base 9f6c0f2 ():

CANCELLED JOB - The following job was cancelled. Please retry:

• trunk / unittest-release / macos / macos-job (gh)
  ##[error]The operation was canceled.

BROKEN TRUNK - The following job failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-llava-runner-linux / linux-job (gh) (trunk failure)
  test_llava_export

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