[llvm] [cmake] Pass PYTHON_EXECUTABLE to native builds (PR #163574)

[David Spickett via llvm-commits]

DavidSpickett wrote:

> Ensure that the nested native build uses the same python interpreter as the main build, in case the python that CMake detects first is not the python that the user has specified explicitly.

Please include in the PR description one example of how the user might have specified that Python. So it is a bit more clear and maybe someone grepping through commits trying to solve their own problem in future will recognise the option.

https://github.com/llvm/llvm-project/pull/163574