snowhawk: make dwm default specialisation

hosts/snowhawk/configuration.nix

@@ -8,15 +8,18 @@
   imports = [
     inputs.home-manager.nixosModules.default
     ../../modules
-  ];
 
-  specialisation = {
-    dwm.configuration = {
+    # Default specialisation
+    ({ lib, config, ... }: {
+      config = lib.mkIf (config.specialisation != { }) {
         snowhawk.dwm.enable = true;
 
         home-manager.sharedModules = [{ snowhawk.dunst.enable = lib.mkDefault true; }];
       };
+    })
+  ];
 
+  specialisation = {
     dwl.configuration = {
       snowhawk.dwl.enable = true;