env: fix check whether path is relative to system site packages (#9861)

We must not use `SystemEnv`, which is Poetry's own env, but `env.parent_env`, which is the base env where the system site packages (in the context of the virtual env) are located.

src/poetry/utils/env/virtual_env.py

@@ -3,7 +3,6 @@
 import json
 import os
 import re
-import sys
 
 from contextlib import contextmanager
 from copy import deepcopy
@@ -17,7 +16,6 @@
 from poetry.utils.env.script_strings import GET_ENVIRONMENT_INFO
 from poetry.utils.env.script_strings import GET_PATHS
 from poetry.utils.env.script_strings import GET_SYS_PATH
-from poetry.utils.env.system_env import SystemEnv
 
 
 if TYPE_CHECKING:
@@ -145,5 +143,5 @@ def includes_system_site_packages(self) -> bool:
     def is_path_relative_to_lib(self, path: Path) -> bool:
         return super().is_path_relative_to_lib(path) or (
             self.includes_system_site_packages
-            and SystemEnv(Path(sys.prefix)).is_path_relative_to_lib(path)
+            and self.parent_env.is_path_relative_to_lib(path)
         )

tests/utils/env/test_env.py

@@ -1,9 +1,7 @@
 from __future__ import annotations
 
-import contextlib
 import os
 import re
-import site
 import subprocess
 import sys
 
@@ -297,16 +295,31 @@ def test_env_system_packages_are_relative_to_lib(
         path=venv_path, flags={"system-site-packages": with_system_site_packages}
     )
     env = VirtualEnv(venv_path)
-    site_dir = Path(site.getsitepackages()[-1])
+
+    # These are Poetry's own dependencies.
+    # They should not be relative to the virtualenv's lib directory.
     for dist in metadata.distributions():
-        # Emulate is_relative_to, only available in 3.9+
-        with contextlib.suppress(ValueError):
-            dist._path.relative_to(site_dir)  # type: ignore[attr-defined]
-            break
-    assert (
-        env.is_path_relative_to_lib(dist._path)  # type: ignore[attr-defined]
-        is with_system_site_packages
-    )
+        assert not env.is_path_relative_to_lib(
+            Path(str(dist._path))  # type: ignore[attr-defined]
+        )
+        # Checking one package is sufficient
+        break
+    else:
+        pytest.fail("No distributions found in Poetry's own environment")
+
+    # These are the virtual environments' base env packages,
+    # in this case the system site packages.
+    for dist in metadata.distributions(path=[str(env.parent_env.site_packages.path)]):
+        assert (
+            env.is_path_relative_to_lib(
+                Path(str(dist._path))  # type: ignore[attr-defined]
+            )
+            is with_system_site_packages
+        )
+        # Checking one package is sufficient
+        break
+    else:
+        pytest.fail("No distributions found in the base environment of the virtualenv")
 
 
 @pytest.mark.parametrize(