Update to use tar-unix 3

[shonfeder]

tar-unix 3 added significant breaking API changes (see ocaml/opam-repository#26330) At this point there do not appear to be any critical fixes of perf benefits that would make an update critical for us (we are not using Tar.Header.marshal afaict, which is the only fix).

But it would be good to update to the new API sooner than later so we can take advantage of critical improvements/fixes when they arrive.