this derivation will be built:
  /nix/store/2h34b3a1i51zjmmyc05c0aicb0kwa03y-interpolate-conformance.drv
bld4: building 2h34b3a1i51zjmmyc05c0aicb0kwa03y-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected