Update cloudbuild-library-generation-integration-test.yaml

Author: blakeli0

.cloudbuild/library_generation/cloudbuild-library-generation-integration-test.yaml

@@ -20,7 +20,8 @@ steps:
   args: [
       "build",
       "--no-cache",
-      "--build-arg", "PROTOC_VERSION=33.2",
+      # Use a fixed protoc version so that the generation result stays the same
+      "--build-arg", "PROTOC_VERSION=25.8",
       "-t", "${_TEST_IMAGE}",
       "-f", ".cloudbuild/library_generation/library_generation_airlock.Dockerfile",
       "."