this derivation will be built:
  /nix/store/3k58c072xpjlgnsr72fqs9kg0wiwj95f-interpolate-conformance.drv
bld4: building 3k58c072xpjlgnsr72fqs9kg0wiwj95f-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected