refactor(nixos): inline module enable conditions

configs/nixos/modules/desktop/utilities/osd.nix

@@ -6,14 +6,13 @@
 }:
 # SwayOSD: a transient on-screen popup for volume and brightness changes.
 let
-  cfg = config.modules.desktop.utilities.osd; # NOLINT(let_order)
   user = config.user.name;
 in
 {
   options.modules.desktop.utilities.osd.enable =
     lib.mkEnableOption "the SwayOSD volume and brightness popup";
 
-  config = lib.mkIf cfg.enable {
+  config = lib.mkIf config.modules.desktop.utilities.osd.enable {
     environment.systemPackages = [ pkgs.swayosd ];
 
     # The udev rule chgrps each backlight's brightness node to `video` and adds