this derivation will be built:
  /nix/store/y9a47v249kv6fdj3nlpcwc835d5icvr3-interpolate-conformance.drv
bld4: building y9a47v249kv6fdj3nlpcwc835d5icvr3-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected