chore: use keystore reference for secrets (#3709)

Author: diegomarquezp

kokoro/ubuntu/release.sh

@@ -19,6 +19,9 @@ if [ -z "$CLOUDSDK_PYTHON" ]; then
 fi
 gcloud components install app-engine-java --quiet
 
+OAUTH_CLIENT_ID=$KOKORO_KEYSTORE_DIR/${KEYSTORE_CONFIG_ID}_CLIENT_ID
+OAUTH_CLIENT_SECRET=$KOKORO_KEYSTORE_DIR/${KEYSTORE_CONFIG_ID}_CLIENT_SECRET
+FIRELOG_API_KEY=$KOKORO_KEYSTORE_DIR/${KEYSTORE_CONFIG_ID}_FIRELOG_API_KEY
 
 # Exit if undefined (zero-length).
 [[ -n "${OAUTH_CLIENT_ID}" ]]