rke2: set the meta.mainProgram (#250748)

Changed files
+1
pkgs
applications
networking
cluster
+1
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";
};
}