this derivation will be built:
  /nix/store/a9v6h4xsm9lsy8jb2cl49gppya8fc4fl-interpolate-conformance.drv
bld3: building a9v6h4xsm9lsy8jb2cl49gppya8fc4fl-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected