refactor(system): simplify Nix config file

system/modules/system/nix.nix

@@ -19,9 +19,9 @@ in
       default = "30d";
     };
   };
-  config = lib.mkMerge [
-    (lib.mkIf cfg.allowUnfree { nixpkgs.config.allowUnfree = true; })
-    ({
+  config = {
+    nixpkgs.config.allowUnfree = cfg.allowUnfree;
+
     nix = {
       # Enable Flakes
       settings.experimental-features = [
@@ -46,6 +46,5 @@ in
 
     # Support for standard, dynamically-linked executables
     programs.nix-ld.enable = true;
-    })
-  ];
+  };
 }