this derivation will be built:
  /nix/store/g73w9sg79fh6jh11zzxjy47rd6zgm9xr-interpolate-conformance.drv
bld4: building g73w9sg79fh6jh11zzxjy47rd6zgm9xr-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected