build.sh: let sbuild manage the tarball in unshare mode

Hi Rust team,

if you find the time, I'd like you to test a new feature of sbuild 0.87.0~exp1 as it currently was uploaded to experimental. Essentially what I added was an ability to sbuild to manage the chroot tarballs itself. This means that the user no longer has to manually create and then continuously recreate (or upgrade) the tarball when using unshare mode.

As you can see from the diff, what this means for you, is that debcargo-conf users now have to do less things as sbuild is being instructed what to do automatically.

I'd value your feedback. Thank you!