Fix the detection of the system environment with custom installer

install-poetry.py

@@ -528,6 +528,10 @@ class Installer:
 
             virtualenv.cli_run([str(env_path), "--clear"])
 
+        # We add a special file so that Poetry can detect
+        # its own virtual environment
+        env_path.joinpath("poetry_env").touch()
+
         return env_path
 
     def make_bin(self, version: str) -> None:

poetry/utils/env.py

@@ -1011,8 +1011,13 @@ class EnvManager:
         """
         prefix, base_prefix = Path(sys.prefix), Path(cls.get_base_prefix())
         if not naive:
+            if prefix.joinpath("poetry_env").exists():
+                return GenericEnv(base_prefix)
+
+            from poetry.locations import data_dir
+
             try:
-                Path(__file__).relative_to(prefix)
+                prefix.relative_to(data_dir())
             except ValueError:
                 pass
             else: