The grpc-tools gem is just a bundle of prebuilt binaries with a thin
layer of wiring to invoke the correct binary given the calling system.
On Nix, prebuilt binaries don't work because they can't find their
dynamically linked libraries in the "normal" places you'd expect on a
Linux machine. Nix has tooling (`autoPatchelf`) which can fixup a given
binary and wire it correctly to the Nix store. We need to have Nix fetch
and build `grpc-tools` so we can invoke this tooling rather than just
letting `bundle install` get the gem.
