this derivation will be built:
  /nix/store/r5c33bfhvhhxdsh994rxlz1i9kwizz48-interpolate-conformance.drv
bld4: building r5c33bfhvhhxdsh994rxlz1i9kwizz48-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected