chore(python): conditionally load credentials in .kokoro/build.sh#798

parthea merged 2 commits intomaingoogleapis/python-api-core:mainfrom

owl-bot-update-lock-f016446d6e520e5fb552c45b110cba3f217bffdd3d06bdddd076e9e6d13266cfgoogleapis/python-api-core:owl-bot-update-lock-f016446d6e520e5fb552c45b110cba3f217bffdd3d06bdddd076e9e6d13266cfCopy head branch name to clipboard