nix-shell --pure repro.nix
succeeds:
$ nix-shell --pure repro.nix
[nix-shell]$ out=final.img
[nix-shell]$ genericBuild
90792
Checking that no-one is using this disk right now ... OK
Disk out.img: 44.3 MiB, 46485504 bytes, 90792 sectors
Units: sectors of 1 * 512 = 512 bytes