(like the installer has) rather than a virtio mount of the host's entire
/nix/store.
-**Update:** `virtualisation.useNixStoreImage` exists now! But it doesn't
-work! :( See the note in `checks/mount-grep.nix`
+**Update:** `virtualisation.useNixStoreImage` exists now! But it builds
+the store image at runtime, rather that at build-time, drastically
+increasing VM start-up time.