this derivation will be built:
  /nix/store/j6lnxqkvm3xvj9372d9fn173xnfrwjj0-interpolate-conformance.drv
bld4: building j6lnxqkvm3xvj9372d9fn173xnfrwjj0-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected