In case of pure input-addressed derivations, the build loop doesn't
guaranty that the realisations are stored in the db (if the output paths
are already there or can be substituted, the realisations won't be
registered). This caused `nix shell` to fail in some cases because it
was assuming that the realisations were always existing.
A better (but more involved) fix would probably to ensure that we always
register the realisations, but in the meantime this patches the surface
issue.
Fix#4721
regnat
8d66f5f110