Pass system through project nix files

default.nix

@@ -1,4 +1,4 @@
-args@{ pkgs ? import ./nix args, ... }:
+args@{ pkgs ? import ./nix args, system ? null, ... }:
 
 {
   inherit (pkgs) arion tests;

nix/default.nix

@@ -1,8 +1,12 @@
 /**
  * This is the entry-point for all nix execution in this project.
  */
-{ nixpkgsSrc ? ./nixpkgs.nix, ... }:
-import (import ./nixpkgs.nix) {
+{ nixpkgsSrc ? ./nixpkgs.nix
+, system ? null
+, ...
+}:
+
+import (import ./nixpkgs.nix) ({
   # Makes the config pure as well. See <nixpkgs>/top-level/impure.nix:
   config = {
   };
@@ -10,4 +14,6 @@ import (import ./nixpkgs.nix) {
     # all the packages are defined there:
     (import ./overlay.nix)
   ];
-}
+} // (if system == null then {} else {
+  inherit system;
+}))

shell.nix

@@ -1 +1 @@
-(import ./nix {}).arion-project.shell
+args@{...}: (import ./nix args).arion-project.shell