this derivation will be built:
  /nix/store/6h40p74kcgzr9zp4c4vcpyk6n9wk43zz-interpolate-conformance.drv
bld4: building 6h40p74kcgzr9zp4c4vcpyk6n9wk43zz-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected