Fix nix run

It requires an explicit `nixpkgs=` entry in `NIX_PATH` to work.

modules/default.nix

@@ -73,6 +73,7 @@
     nix = {
       nixPath = [
         "/var/src" # pinned nixpkgs and configuration
+        "nixpkgs=/var/src/nixpkgs" # for nix run
       ];
       # Make sudoers trusted nix users
       trustedUsers = [ "@wheel" ];