this derivation will be built:
  /nix/store/i3di6narpbwm5c2a8g94ayinvn0ycifk-interpolate-conformance.drv
bld4: building i3di6narpbwm5c2a8g94ayinvn0ycifk-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected