build: Add protobuf dependencies to Nix (#7706)

Co-authored-by: Bart <11445373+bthomee@users.noreply.github.com>

nix/packages.nix

@@ -32,6 +32,15 @@ in
     perl # needed for openssl
     pkg-config
     pre-commit
+    # protoc generates the Go gRPC bindings and embeds its own version string into every committed
+    # .pb.go file. To allow CI to verify those files with a plain `git diff`, we pin the version to
+    # `protobuf_34` rather than the rolling `protobuf` to keep regeneration reproducible across the
+    # Nix frequently changing unstable channel. The protoc-gen-go* plugins have no versioned
+    # attributes in nixpkgs; protoc-gen-go's version is in turn constrained by the go.mod require
+    # on google.golang.org/protobuf.
+    protobuf_34 # provides protoc
+    protoc-gen-go # protoc plugin for the Go message bindings
+    protoc-gen-go-grpc # protoc plugin for the Go gRPC service stubs
     python3
     runClangTidy
     vim