The runner mounts a persistent nix-store volume over /nix, and the
nixos/nix image makes /etc/nix/nix.conf a symlink into /nix/store. When
the volume and image versions differ, the link dangles and appending to
it fails before checkout. That broke asxpio and datacenterbingo CI on
2026-10-18.
Pass the settings in NIX_CONFIG instead, with build-users-group empty
and the sandbox off as the image config has them.
Assisted-by: Claude:opus-5-5
