refactor: Fleet and sw behind mkIf guards

sw/update-ref.nix

@@ -1,6 +1,18 @@
-{ pkgs, ... }:
 {
-  environment.systemPackages = with pkgs; [
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+
+with lib;
+
+let
+  cfg = config.athenix.sw;
+in
+{
+  config = mkIf cfg.enable {
+    environment.systemPackages = with pkgs; [
     python3
     git
     (pkgs.writeShellScriptBin "update-ref" ''
@@ -508,4 +520,5 @@
       printf "  rev = %s\n" "$CUR_REV" >&2
     '')
   ];
+  };
 }