this derivation will be built:
  /nix/store/rvfisdgzf8xq7s2sly7sspllv5xw4157-interpolate-conformance.drv
bld3: building rvfisdgzf8xq7s2sly7sspllv5xw4157-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected