fix pure nix evaluation

There was access to impure builtins in the nix evaluation
so pure evaluation was not possible.

Author: dermetfan

nix/default.nix

@@ -1,3 +1,3 @@
 { ... }@args:
 with (import ./flake-compat.nix args);
-defaultNix.legacyPackages.${builtins.currentSystem}
+defaultNix.legacyPackages.${args.system or builtins.currentSystem}

nix/nixos/cardano-node-service.nix

@@ -183,7 +183,7 @@ in {
 
       cardanoNodePackages = mkOption {
         type = types.attrs;
-        default = pkgs.cardanoNodePackages or (import ../. {}).cardanoNodePackages;
+        default = pkgs.cardanoNodePackages or (import ../. { inherit (pkgs) system; }).cardanoNodePackages;
         defaultText = "cardano-node packages";
         description = ''
           The cardano-node packages and library that should be used.