pin nixos-wide registry to nixpkgs used to build the machine itself

Instead of downloading archives and filling up vm disk space we can juse the nixpkgs version we already use for evaluating

lib/build-clan/default.nix

@@ -43,6 +43,12 @@ let
           clanCore.machineName = name;
           clanCore.clanDir = directory;
           nixpkgs.hostPlatform = lib.mkForce system;
+
+          # speeds up nix commands by using the nixpkgs from the host system (especially useful in VMs)
+          nix.registry.nixpkgs.to = {
+            type = "path";
+            path = lib.mkDefault nixpkgs;
+          };
         }
       ];
     inherit specialArgs;