chore: fix conformance test failure due to VM's API version mismatch. (#1635)

Conformance tests were failing due to Docker API version mismatch in the
kokoro's VM and docker client.

This PR pins the `DOCKER_API_VERSION` to 1.39

RCA for kokoro failures -
https://screenshot.googleplex.com/4zsxoQ8UxqWnTky.

`Error response from daemon: client version 1.52 is too new. Maximum
supported API version is 1.39`

Author: chandra-siri

noxfile.py

@@ -232,7 +232,7 @@ def conftest_retry(session):
         test_cmd = ["py.test", "-n", "auto", "--quiet", conformance_test_folder_path]
 
     # Run py.test against the conformance tests.
-    session.run(*test_cmd)
+    session.run(*test_cmd, env={"DOCKER_API_VERSION": "1.39"})
 
 
 @nox.session(python=DEFAULT_PYTHON_VERSION)