gh-91985: Ensure the same path calculations when repeated with PYTHONHOME

[neonene]

When an embedded Python is initialized multiple times, getpath.py requires home with the same value, for the consistent path configuration.

Roughly, the transition of home (PyInterpreterState.config.home) is as below:

1. home = config.home(original, local) or _Py_path_config.home(global)
2. home, paths = getpath(home, PYTHONHOME, *._pth)
3. _Py_path_config.home = home (the original config.home is left as-is)

1st initialization with PYTHONHOME:

1. home gets NULL.
2. getpath receives None as home and returns the value of PYTHONHOME.

2nd:

1. home starts with PYTHONHOME from _Py_path_config.home.
2. getpath keeps the existing home unchanged. The returned paths, which are less flexible than 1st, can cause import errors.

In this cycle, the value from _Py_path_config.home at step1 needs to be discarded unless the global value was set by Py_SetPythonHome().

#91985

[neonene]

@zooba Could you take a look please?