this derivation will be built:
  /nix/store/fv5mv473kbi89wd0d1ny2nqc0mjs4m8c-interpolate-conformance.drv
bld4: building fv5mv473kbi89wd0d1ny2nqc0mjs4m8c-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected