refactor(nixos): inline module enable conditions

configs/nixos/modules/shell/omp.nix

@@ -6,7 +6,6 @@
 }:
 # Oh My Pi, installed alongside upstream Pi as the `omp` command.
 let
-  cfg = config.modules.shell.omp; # NOLINT(let_order)
   user = config.user.name;
 in
 {
@@ -14,7 +13,7 @@ in
     enable = lib.mkEnableOption "Oh My Pi, a batteries-included fork of Pi";
   };
 
-  config = lib.mkIf cfg.enable {
+  config = lib.mkIf config.modules.shell.omp.enable {
     home-manager.users.${user}.home.packages = [ pkgs.omp ];
   };
 }