this derivation will be built:
  /nix/store/fq2biif5xk4bxzshf7w0i9dg93i5q20z-interpolate-conformance.drv
bld4: building fq2biif5xk4bxzshf7w0i9dg93i5q20z-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected