use flake.nixosConfigurations.thing.extendModules instead of configuration pattern builder

[maxhbr]

some example:

  ┃  $ nix eval --impure --expr '                                                                                                                                                   
  ┃  let                                                                                                                                                                            
  ┃    flake = builtins.getFlake "git+file:///home/mhuber/myconfig/myconfig";                                                                                                       
  ┃    cfg = flake.nixosConfigurations.thing.extendModules {                                                                                                                        
  ┃      modules = [ { myconfig.ai.aider.enable = true; } ];                                                                                                                        
  ┃    };                                                                                                                                                                           
  ┃  in                                                                                                                                                                             
  ┃  cfg.config.system.build.toplevel.drvPath                                                                                                                                       
  ┃  ' 2>&1 | tail -20