fix extra-container eval: add dummy nixos-init option
extra-container uses a minimal NixOS module set for fast evaluation.
nixpkgs unstable added nixos-init.nix and systemd.nix now references
config.system.nixos-init.package, but extra-container's module set
doesn't include it.
Add a dummy nixos-init.package option to the outer container config,
matching extra-container's existing pattern for missing options.
Also fixes services.mingetty -> services.getty rename in virtulus.