this derivation will be built:
  /nix/store/rq7wd96qsjyfr1mg4gy3jf4j8apcc7jz-interpolate-conformance.drv
bld4: building rq7wd96qsjyfr1mg4gy3jf4j8apcc7jz-interpolate-conformance.drv
10 cases: Nix interpolate == Python interpolate == expected