modules: system: nix: simplify 'NIX_PATH'

Since we now have an explicit 'pkgs' link, we can just add the folder
with all linked inputs directly instead of adding them all manually.

modules/system/nix/default.nix

@@ -73,11 +73,8 @@ in
 
     (lib.mkIf cfg.addToNixPath {
       nix.nixPath = [
-        "self=/etc/nix/inputs/self"
+        "/etc/nix/inputs"
-        "pkgs=/etc/nix/inputs/pkgs"
-        "nur=/etc/nix/inputs/nur"
       ]
-      ++ lib.optional cfg.overrideNixpkgs "nixpkgs=/etc/nix/inputs/nixpkgs"
       ++ options.nix.nixPath.default;
     })
   ]);