ci(kokoro): fix NOX_FILE and NOX_SESSION override for google-auth in system.sh

Author: ohmayr

.kokoro/system.sh

@@ -60,8 +60,13 @@ run_package_test() {
 
       PROJECT_ID=$(cat "${KOKORO_GFILE_DIR}/google-auth-project-id.json")
       GOOGLE_APPLICATION_CREDENTIALS="${KOKORO_GFILE_DIR}/google-auth-service-account.json"
-      NOX_FILE="system_tests/noxfile.py"
-      NOX_SESSION=""
+      if [[ -z "${NOX_SESSION}" || "${NOX_SESSION}" == "system-"* ]]; then
+        NOX_FILE="system_tests/noxfile.py"
+        NOX_SESSION=""
+      else
+        NOX_FILE="noxfile.py"
+        NOX_SESSION="${NOX_SESSION}"
+      fi
       ;;
     *)
       PROJECT_ID=$(cat "${KOKORO_GFILE_DIR}/project-id.json")