env: remove leaky vars from pep517 environ

Closes pypa#377.

src/build/env.py

@@ -185,6 +185,16 @@ class _DefaultIsolatedEnv(IsolatedEnv):
     An isolated environment class which combines venv and virtualenv with pip.
     """
 
+    _LEAKY_ENV_VARS = frozenset(
+        {
+            'PYTHONHOME',
+            'PYTHONPATH',
+            'PYTHONPLATLIBDIR',
+            'PYTHONSTARTUP',
+            'PYTHONNOUSERSITE',
+        }
+    )
+
     def __init__(self, isolated_env_manager: 'IsolatedEnvManager') -> None:
         self._isolated_env_manager = isolated_env_manager
         self._log = self._isolated_env_manager.log
@@ -247,6 +257,8 @@ def install_packages(self, requirements: Iterable[str]) -> None:
 
     def prepare_environ(self) -> Dict[str, str]:
         environ = os.environ.copy()
+        for name in self._LEAKY_ENV_VARS:
+            environ.pop(name, None)
 
         # Make the virtual environment's scripts available to the project's build dependecies.
         path = environ.get('PATH')