this derivation will be built:
  /nix/store/gwg6db8lz2fm8kcfx6b6jaklqx8s5bsp-interpolate-conformance.drv
bld4: building gwg6db8lz2fm8kcfx6b6jaklqx8s5bsp-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected