this derivation will be built:
  /nix/store/h8zsv9in5nj2kcmqn3xn4y4p1s6rxn70-interpolate-conformance.drv
bld3: building h8zsv9in5nj2kcmqn3xn4y4p1s6rxn70-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected