feat(tools): Add 'gotest' program to demonstrate pkgs.buildGo

This is a tiny program that does nothing but exists to demonstrate
pkgs.buildGo by building a program that depends on a local library as
well as a protobuf definition.

tools/gotest/default.nix

@@ -0,0 +1,27 @@
+# This file demonstrates how to make use of pkgs.buildGo.
+#
+# It introduces libraries and protobuf support, however gRPC support
+# is not yet included.
+#
+# From the root of this repository this example can be built with
+# `nix-build -A tools.gotest`
+{ pkgs, ... }:
+
+let
+  inherit (pkgs) buildGo;
+
+  somelib = buildGo.package {
+    name = "somelib";
+    srcs = [ ./lib.go ];
+  };
+
+  someproto = buildGo.proto {
+    name = "someproto";
+    proto = ./test.proto;
+  };
+
+in buildGo.program {
+  name = "gotest";
+  srcs = [ ./main.go ];
+  deps = [ somelib someproto ];
+}