gh-128779: Fix system site-packages test inside a virtual environment

Use the base interpreter prefixes when expanding the system install scheme so nested virtual environments are compared with the base interpreter site-packages rather than the outer virtual environment.

Co-authored-by: Muse Code (Meta Muse Spark)

Author: ambv

Lib/test/test_site.py

@@ -618,9 +618,13 @@ def test_system_site_packages(self):
             ))
         """)
 
-        # Use _get_preferred_schemes to find the system scheme, in case we are in a virtual environment
+        # Use the base interpreter's scheme and prefixes, even when running
+        # inside a virtual environment.
         system_scheme_name = sysconfig._get_preferred_schemes()['prefix']
-        system_paths = sysconfig.get_paths(system_scheme_name)
+        system_paths = sysconfig.get_paths(system_scheme_name, vars={
+            'base': sys.base_prefix,
+            'platbase': sys.base_exec_prefix,
+        })
 
         with self.venv(system_site_packages=False) as venv:
             sys_path = json.loads(venv.run('-c', script).stdout)