this derivation will be built:
  /nix/store/phbd6hvjfnrsnxfngzac1isza6h9zndp-interpolate-conformance.drv
bld4: building phbd6hvjfnrsnxfngzac1isza6h9zndp-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected