this derivation will be built:
  /nix/store/crznrsl8xxbri563mz5ixnqlkxvfvpj6-interpolate-conformance.drv
bld3: building crznrsl8xxbri563mz5ixnqlkxvfvpj6-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected