GH-126985: Don't override venv detection with PYTHONHOME

[FFY00]

PYTHONHOME should only override the base prefix calculation, it should have no effect on detecting if we are running inside a virtual environment.

• Issue: Make pyvenv.cfg-relocation part of the interpreter initialization instead of the site module #126985

[FFY00]

Without the fix prefix and exec_prefix would be set to PYTHONHOME:

test test_getpath failed -- Traceback (most recent call last):
  File "/home/anubis/git/cpython/Lib/test/test_getpath.py", line 865, in test_PYTHONHOME_in_venv
    self.assertEqual(expected, actual)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
AssertionError: {'exe[37 chars]': '/venv', 'exec_prefix': '/venv', 'base_pref[206 chars]ad']} != {'exe[37 chars]': '/pythonhome', 'exec_prefix': '/pythonhome'[218 chars]ad']}
  {'base_exec_prefix': '/pythonhome',
   'base_prefix': '/pythonhome',
-  'exec_prefix': '/venv',
?                   ^ --

+  'exec_prefix': '/pythonhome',
?                   ^^^^^^^^^

   'executable': '/venv/bin/python',
   'module_search_paths': ['/pythonhome/lib/python98.zip',
                           '/pythonhome/lib/python9.8',
                           '/pythonhome/lib/python9.8/lib-dynload'],
   'module_search_paths_set': 1,
-  'prefix': '/venv'}
+  'prefix': '/pythonhome'}