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.

tools/run/.envrc

@@ -1,2 +1 @@
 source_up
-eval "$(lorri direnv)"

tools/run/dir-locals.nix

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

tools/run/shell.nix

@@ -1,7 +1,9 @@
-{ pkgs, ... }:
-
-pkgs.mkShell {
+let
+  pkgs = import <nixpkgs> {};
+in pkgs.mkShell {
   buildInputs = with pkgs; [
     go
+    goimports
+    godef
   ];
 }