gh-155491: Fix subprocess test_encoding_warning() (GH-155492)

The test fails if run using PYTHONWARNINGS=error environment
variable. Use make_clean_env() function to not inherit PYTHON
environment variables.
(cherry picked from commit 433c842)

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: vstinner

Lib/test/test_subprocess.py

@@ -2051,8 +2051,9 @@ def test_encoding_warning(self):
             run("echo hello", shell=True, text=True)
             check_output("echo hello", shell=True, text=True)
             """)
+        env = support.make_clean_env()
         cp = subprocess.run([sys.executable, "-Xwarn_default_encoding", "-c", code],
-                            capture_output=True)
+                            capture_output=True, env=env)
         lines = cp.stderr.splitlines()
         self.assertEqual(len(lines), 2, lines)
         self.assertStartsWith(lines[0], b"<string>:2: EncodingWarning: ")