this derivation will be built:
  /nix/store/f869gm65fblm87rp3bslshc9ma8li58b-interpolate-conformance.drv
bld4: building f869gm65fblm87rp3bslshc9ma8li58b-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected