fix tar feature in gix-archive

[gruebel]

Currently it is not possible to only choose the tar feature in gix-archive, also Format::TarGz was not correctly instantiated and throw an additional error.

[Byron]

Thanks a lot for the fix, it's much appreciated.

Unfortunately, I can't merge this without a test that triggers it, ideally in a separate commit and push, so CI can turn green with the fix.

Let's fix the reason this bug was present - the missing test.
Thanks again 🙏.

[gruebel]

@Byron
I checked and actually there is already a test for it, but the problem was the test run for gix-archive were not correctly setup and I fixed in the justfile. I also had to enable the deflate-flate2-zlib-rs feature in zip for the gzip feature, otherwise it complains not having a backend. let me know, if it works for you.

[Byron]

That's great, thanks so much for squelching this long-standing bug :).