CMAKE_INSTALL_PREFIX not honored for Python files installation on Windows

[ricrogz]

This is a second fix after #6339. Even after #6371, CMAKE_INSTALL_PREFIX is not honored when building under windows. I just built 2023.03.2, and noticed that the Python wrappers were not installed under the path I had specified.

The cause of the issue is that the "nt" scheme in sysconfig.get_path() uses the base key for the "platlib" path instead of platbase, as does the "posix_prefix" schema:

In [9]: sysconfig._INSTALL_SCHEMES
Out[9]:
{'posix_prefix': {'stdlib': '{installed_base}/lib/python{py_version_short}',
  'platstdlib': '{platbase}/lib/python{py_version_short}',
  'purelib': '{base}/lib/python{py_version_short}/site-packages',
  'platlib': '{platbase}/lib/python{py_version_short}/site-packages',
  'include': '{installed_base}/include/python{py_version_short}{abiflags}',
  'platinclude': '{installed_platbase}/include/python{py_version_short}{abiflags}',
  'scripts': '{base}/bin',
  'data': '{base}'},
[...]
 'nt': {'stdlib': '{installed_base}/Lib',
  'platstdlib': '{base}/Lib',
  'purelib': '{base}/Lib/site-packages',
  'platlib': '{base}/Lib/site-packages',
  'include': '{installed_base}/Include',
  'platinclude': '{installed_base}/Include',
  'scripts': '{base}/Scripts',
  'data': '{base}'},
[...]
}

Nice inconsisteny, Python...

[kuelumbus]

This breaks the builds for the windows python wheels. Will cherry-pick this fix.

[greglandrum]

LGTM