this derivation will be built:
  /nix/store/hk97i9akk4vk14sfcnk0aa6z3h5xhi7d-interpolate-conformance.drv
bld4: building hk97i9akk4vk14sfcnk0aa6z3h5xhi7d-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected