nixos/i3: fix i3lock default enable-ing

Prior to this commit, i3lock was enabled by default in a way that doesn't work as of 317c972e8a (diff-aef862f6fd2c25092a3f17f974d8757285bf7baff6b80822cd142b7de1903ccfR451-R454)
Users attempting to use i3lock with this default setup would get locked out of their system.

This fixes it by enabling i3lock via its `programs` option instead of specifying the package directly.

Discussion over at https://github.com/NixOS/nixpkgs/issues/401891#issuecomment-2963378189

nixos/modules/services/x11/window-managers/i3.nix

@@ -52,13 +52,11 @@ in
       default = with pkgs; [
         dmenu
         i3status
-        i3lock
       ];
       defaultText = literalExpression ''
         with pkgs; [
           dmenu
           i3status
-          i3lock
         ]
       '';
       description = ''
@@ -81,6 +79,7 @@ in
         '';
       }
     ];
+    programs.i3lock.enable = mkDefault true;
     environment.systemPackages = [ cfg.package ] ++ cfg.extraPackages;
     environment.etc."i3/config" = mkIf (cfg.configFile != null) {
       source = cfg.configFile;