Define PYTHON_EXECUTABLE only once in cmake (#10911)

### 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