this derivation will be built:
  /nix/store/kjkxzqka6r7b8mxvc6fl9gzz06rgc664-interpolate-conformance.drv
bld3: building kjkxzqka6r7b8mxvc6fl9gzz06rgc664-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected