This command is used in `go generate` but hadn't been pulled into the
nix dependencies yet. Previously I was installing the binary via `go
get` but this is cleaner and more nix-y.
Also includes a `nix flake update` for good measure.
Keeping things pinned to the last released version should hopefully help
minimize comment churn (as each generated run outputs the version in the
comments).
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.
