Pin Nixpkgs from flake to registry in HM config

nix/flake.nix

@@ -27,6 +27,9 @@
           home = {
             inherit username;
           };
+          # Pin nixpkgs flake to local nix registry so it gets used by commands like nix-shell
+          # https://discourse.nixos.org/t/local-flake-based-nix-search-nix-run-and-nix-shell/13433/12
+          nix.registry.nixpkgs.flake = nixpkgs;
         }
       ];
     });