this derivation will be built:
  /nix/store/9ck4j93k5ycy5gdmfhmdcr7nzk2zknri-interpolate-conformance.drv
bld4: building 9ck4j93k5ycy5gdmfhmdcr7nzk2zknri-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected