The VM can attempt to be built with nix-build -A myMachine.config.system.build.vm.

Currently this depends on using a WIP branch of ngipkgs https://github.com/ngi-nix/ngipkgs/tree/default-nix-init created as part of a PR ngi-nix/ngipkgs#209 to add a default.nix to the repo.