this derivation will be built:
  /nix/store/7dcqc2g8283m2j7bw3grb64ip1yl82kd-interpolate-conformance.drv
bld4: building 7dcqc2g8283m2j7bw3grb64ip1yl82kd-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected