this derivation will be built:
  /nix/store/jc4xfy4ldzg1x4drqn7c7p4kcn5crgim-interpolate-conformance.drv
bld4: building jc4xfy4ldzg1x4drqn7c7p4kcn5crgim-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected