Missing `time` and `nixos-init` attributes when using latest nixpkgs-unstable

[adamjhf]

Using extra-containers with latest nixpkgs-unstable fails to build due to missing options:

• Missing config.time.timeZone
  • referenced by nixpkgs nixos/modules/system/boot/systemd.nix:825
  • error: attribute 'time' missing
• Missing config.system.nixos-init.package
  • referenced by nixpkgs nixos/modules/system/boot/systemd.nix:668
  • error: attribute 'nixos-init' missing