nix: fmt + remove pkgs.system ref

nix/nixos-module.nix

@@ -13,10 +13,21 @@
 # RT-priority tuning — use the Home Manager module
 # (`homeModules.default`) instead. The two compose.
 self:
-{ config, lib, pkgs, ... }:
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
 
 let
-  inherit (lib) mkEnableOption mkOption mkIf types literalExpression;
+  inherit (lib)
+    mkEnableOption
+    mkOption
+    mkIf
+    types
+    literalExpression
+    ;
 
   cfg = config.programs.headroom;
 in
@@ -26,7 +37,7 @@ in
 
     package = mkOption {
       type = types.package;
-      default = self.packages.${pkgs.system}.headroom;
+      default = self.packages.${pkgs.stdenv.hostPlatform.system}.headroom;
       defaultText = literalExpression "headroom.packages.\${pkgs.system}.headroom";
       description = ''
         The headroom package to install system-wide.