this derivation will be built:
  /nix/store/7cdf5dbjgc7ghj8h6iviavimy6147gwm-interpolate-conformance.drv
bld4: building 7cdf5dbjgc7ghj8h6iviavimy6147gwm-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected