this derivation will be built:
  /nix/store/fjj1giin2gkh77jv3f1cz06c6giv0r7x-interpolate-conformance.drv
bld4: building fjj1giin2gkh77jv3f1cz06c6giv0r7x-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected