this derivation will be built:
  /nix/store/kp72c47961lvw7k8b9d28y0nkzn0smf9-interpolate-conformance.drv
bld4: building kp72c47961lvw7k8b9d28y0nkzn0smf9-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected