this derivation will be built:
  /nix/store/d3xw97zscdd7ns9m0mkj42arjlxgkw13-interpolate-conformance.drv
bld3: building d3xw97zscdd7ns9m0mkj42arjlxgkw13-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected