this derivation will be built:
  /nix/store/fxi9rj6pzwdbjg7fd1fx8rj6ph0a985c-interpolate-conformance.drv
bld4: building fxi9rj6pzwdbjg7fd1fx8rj6ph0a985c-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected