gh-115538: Use isolate mode when running venv test_multiprocessing_recursion() (#117116)

Co-authored-by: Victor Stinner <[email protected]>

Author: aisk

Lib/test/test_venv.py

@@ -531,7 +531,7 @@ def test_multiprocessing_recursion(self):
         rmtree(self.env_dir)
         self.run_with_capture(venv.create, self.env_dir)
         script = os.path.join(TEST_HOME_DIR, '_test_venv_multiprocessing.py')
-        subprocess.check_call([self.envpy(real_env_dir=True), script])
+        subprocess.check_call([self.envpy(real_env_dir=True), "-I", script])
 
     @unittest.skipIf(os.name == 'nt', 'not relevant on Windows')
     def test_deactivate_with_strict_bash_opts(self):