this derivation will be built:
  /nix/store/ad7kmf9pr9543xg8jzlycn9pmr1gczw8-interpolate-conformance.drv
bld3: building ad7kmf9pr9543xg8jzlycn9pmr1gczw8-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected