this derivation will be built:
  /nix/store/2k1k98phfanmqc137pzaj5rs6hf701vj-interpolate-conformance.drv
bld4: building 2k1k98phfanmqc137pzaj5rs6hf701vj-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected