this derivation will be built:
  /nix/store/bxy5yivyiqk0byhgxxsn94km5ihywr48-interpolate-conformance.drv
bld4: building bxy5yivyiqk0byhgxxsn94km5ihywr48-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected