this derivation will be built:
  /nix/store/dkc1d2bghiibhqrpmdjcyvp9cksigr44-interpolate-conformance.drv
bld4: building dkc1d2bghiibhqrpmdjcyvp9cksigr44-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected