Add correct handling of GOBIN to example/protogen.sh.
Add correct handling of GOBIN to example/protogen.sh.

improbable-eng/grpc-web,MarcusLongmuir/grpc-web,MarcusLongmuir/grpc-web,MarcusLongmuir/grpc-web,improbable-eng/grpc-web,MarcusLongmuir/grpc-web,improbable-eng/grpc-web,improbable-eng/grpc-web,improbable-eng/grpc-web,MarcusLongmuir/grpc-web