Merge pull request #4792 from Unity-Technologies/proto-build-fixes

Try to fix proto build test

.yamato/protobuf-generation-test.yml

   name: Protobuf Generation Tests
   agent:
     type: Unity::VM::osx
-    image: ml-agents/ml-agents-bokken-mac:0.1.4-492264
+    image: package-ci/mac:stable
+    HOMEBREW_NO_AUTO_UPDATE: "1"
   commands:
     - |
       brew install nuget