This replaces some derivation outputs by simple strings rather than full Nix store paths. This removes the need to download the whole derivation when all we need is a static string. (cherry picked from commit 797c77a00a)
797c77a00a
layout
variant
Also default these options to `null` for state version ≥ 19.09. Fixes #811 Suggested-by: Sean Marshallsay <srm.1708@gmail.com>