Kokoro job grpc/core/master/linux/grpc_build_protobuf_at_head failing on master

The job has been red on kokoro for a while, but it would be good to fix it so it's green most of the times and we don't have that much noise in our master builds.

Also, it looks like there are strange infrastructure failures associated with this jobs (they don't happen elsewhere - except https://github.com/grpc/grpc/issues/14861).