Use monkeypatch in test_pytest_version_env_var

Follow up to #12190

Author: dheerajck

testing/test_runner.py

@@ -1097,7 +1097,7 @@ def func() -> None:
 
 
 def test_pytest_version_env_var(pytester: Pytester, monkeypatch: MonkeyPatch) -> None:
-    os.environ["PYTEST_VERSION"] = "old version"
+    monkeypatch.setenv("PYTEST_VERSION", "old version")
     pytester.makepyfile(
         """
         import pytest