Drop support for lorri

Lorri does not cleanly integrate with my corporate device, which cannot run
NixOS. To expose dependencies to Emacs buffers, I will use nix-buffer.el, which
reads its values from dir-locals.nix. To easily expose dependencies from my
existing shell.nix files into dir-locals.nix, I wrote a Nix utility function.

go/.envrc

@@ -1 +1 @@
-eval "$(lorri direnv)"
+source_up

go/dir-locals.nix

@@ -0,0 +1,3 @@
+let
+  briefcase = import <briefcase> {};
+in briefcase.utils.nixBufferFromShell ./shell.nix

go/shell.nix

@@ -1,6 +1,6 @@
-{ pkgs, ... }:
-
-pkgs.mkShell {
+let
+  pkgs = import <nixpkgs> {};
+in pkgs.mkShell {
   buildInputs = [
     pkgs.go
     pkgs.goimports