this derivation will be built:
  /nix/store/456dw65v5s0hqy0nc8dnc8qjf58gj01m-interpolate-conformance.drv
bld3: building 456dw65v5s0hqy0nc8dnc8qjf58gj01m-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected