this derivation will be built:
  /nix/store/n9rzwawf9h2s43dfdvbwldmd3fbv92ly-interpolate-conformance.drv
bld4: building n9rzwawf9h2s43dfdvbwldmd3fbv92ly-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected