Augment the current environment providing the PYTHONUSERBASE.

def _run_env(self):
        """
        Augment the current environment providing the PYTHONUSERBASE.
        """
        env = dict(os.environ)
        env.update(
            getattr(self, 'env', {}),
            PYTHONUSERBASE=self.env_path,
            PIP_USER="1",
        )
        self._disable_venv(env)
        return env