rename systemPackages as defaultProfile.packages

it doesn't work the same way as in nixos, so don't name it the same way

tests/pppoe/configuration.nix

@@ -68,5 +68,5 @@ in rec {
     ];
   };
 
-  systemPackages = [ pkgs.hello ] ;
+  defaultProfile.packages = [ pkgs.hello ] ;
 }