this derivation will be built:
  /nix/store/bx9f3nszplsijp67x7ynz9zjfqgbir6n-interpolate-conformance.drv
bld4: building bx9f3nszplsijp67x7ynz9zjfqgbir6n-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected