rke2: set the meta.mainProgram (#250748)

pkgs/applications/networking/cluster/rke2/default.nix

···
     changelog = "https://github.com/rancher/rke2/releases/tag/v${version}";
     license = licenses.asl20;
     maintainers = with maintainers; [ zimbatm zygot ];
+    mainProgram = "rke2";
   };
 }