this derivation will be built:
  /nix/store/cvnwm19zdrkf5j6plll1injy9vck3ga1-interpolate-conformance.drv
bld4: building cvnwm19zdrkf5j6plll1injy9vck3ga1-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected