nixos/github-runner{,s}: add @veehaitch and @newam as maintainers

nixos/modules/services/continuous-integration/github-runner.nix

···
   config = mkIf cfg.enable {
     services.github-runners.${cfg.name} = cfg;
   };
+
+  meta.maintainers = with maintainers; [ veehaitch newam ];
 }

nixos/modules/services/continuous-integration/github-runners.nix

···
         }))
     );
   };
+
+  meta.maintainers = with maintainers; [ veehaitch newam ];
 }