I’m not supposed to define it myself, however, it work in nix repl:
nix-repl> :lf .
Added 13 variables.
nix-repl> nixosConfigurations.x86_64-linux.bare.config.system.build.toplevel
«derivation /nix/store/46x1czi4sxh0w3fjyrka1zfdmi7prcmq-nixos-system-ci-23.11.20230810.ce5e4a6.drv»