Prevent the environment from being clobbered during sh_test

diff --git a/tests/sh_test.py b/tests/sh_test.py
index ab41055..d1b08c6 100644
--- a/tests/sh_test.py
+++ b/tests/sh_test.py
@@ -29,12 +29,8 @@ class ShellScriptTests(basetest.TestCase):
     path_with_python = ':'.join((
         os.path.dirname(sys.executable), os.environ.get('PATH')))
 
-    env = {
-        'PATH': path_with_python,
-        # Setuptools puts dependency eggs in our path, so propagate that.
-        'PYTHONPATH': os.pathsep.join(sys.path),
-        'TEST_TMPDIR': FLAGS.test_tmpdir,
-        }
+    env = os.environ.copy()
+    env['TEST_TMPDIR'] = FLAGS.test_tmpdir
     p = subprocess.Popen(sh_test_path, cwd=tests_path, env=env)
     self.assertEqual(0, p.wait())
 
