hetzner-bootstrap: Fix evaluation for latest <nixpkgs>.

The commit NixOS/nixpkgs@b3cfb90 added a lib argument to the function attrset of <nixpkgs/nixos/modules/installer/tools/tools.nix>, so we need to pass it on our side as well.

This change should be backwards-compatible, because the lib argument isn't used for `nixos-generate-config`.