this derivation will be built:
  /nix/store/wvb7raqxh20kjnadplkszwpl5bfa3kmv-interpolate-conformance.drv
bld4: building wvb7raqxh20kjnadplkszwpl5bfa3kmv-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected