Add nix shell environment

.envrc

@@ -0,0 +1 @@
+use nix

shell.nix

@@ -0,0 +1,9 @@
+{ pkgs ? (import <nixpkgs> {}) }:
+
+pkgs.mkShell {
+  buildInputs = with pkgs; [
+    gnumake
+    go
+    gopls
+  ];
+}