this derivation will be built:
  /nix/store/0m60c8fi7qj33ndyg7ik40b66n73nsd1-interpolate-conformance.drv
bld4: building 0m60c8fi7qj33ndyg7ik40b66n73nsd1-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected