[vcpkg|python] python commands being broken by system installed python

[Neumann-A]

Observed Error:; pkgconf fails to build or any python command fails.

with something like:

G:\vcpkg_common\downloads\tools\python>G:\vcpkg_common\downloads\tools\python\python-3.11.5-x64\python.exe
Python 3.11.5 (tags/v3.11.5:cce6ba9, Aug 24 2023, 14:38:34) [MSC v.1936 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
> import _ctypes
> Traceback (most recent call last):
>   File "<stdin>", line 1, in <module>
> ImportError: DLL load failed while importing _ctypes: Das angegebene Modul wurde nicht gefunden.
> >>>

(error only happens in CWD is different from where python lives)

Reason for failure is a globaly installed Python adding:
HKEY_LOCAL_MACHINE\SOFTWARE\Python\PythonCore\3.11\PythonPath
modifying sys.path (dll lookup path) for the python used by vcpkg.

vcpkg probably should add -I (or at least -E) to all python calls to avoid modifications of any kind. (Unfortunatly this also deactivates using PYTHONPATH as an environment variable)

[Osyotr]

Normally, embeddable python package comes with python3xy._pth that makes it isolated from user environment. However, that file was removed in #7051.