gh-150046: Fix test_add_python_opts to ignore PYTHON* env vars (GH-150089)

Avoid the runtime environment from affecting the tests' behaviours,
which notably checks the warning filters which can be controlled by
various PYTHON environment variables.
(cherry picked from commit ef2246f)

Co-authored-by: Pradyun Gedam <pradyunsg@gmail.com>

Author: pradyunsg

Lib/test/test_regrtest.py

@@ -2275,7 +2275,8 @@ def test_python_opts(self):
         proc = subprocess.run(cmd,
                               stdout=subprocess.PIPE,
                               stderr=subprocess.STDOUT,
-                              text=True)
+                              text=True,
+                              env=support.make_clean_env())
         self.assertEqual(proc.returncode, 0, proc)
 
     def test_add_python_opts(self):