this derivation will be built:
  /nix/store/p31927yln5jzpfb0az007h4vzb1izx36-interpolate-conformance.drv
bld4: building p31927yln5jzpfb0az007h4vzb1izx36-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected