stock image, nuget installed (I hope)

.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
     flavor: b1.small
   variables:
     GRPC_VERSION: "1.14.1"
-      brew install nuget
       nuget install Grpc.Tools -Version $GRPC_VERSION -OutputDirectory protobuf-definitions/
       python3 -m venv venv
       . venv/bin/activate