Some users will have `boot.zfs.forceImportRoot = false;` in their
configurations which conflicts with `boot.zfs.forceImportAll = true;`,
so we set it to `true` to match.
As `makeDiskImages` always requires a NixOS configuration, we can
simplify the code by convering it into a NixOS module. Then we can make
it responsible for populating `system.build.diskoImages` and
`system.build.diskoImagesScript`.