It sounds like there's two obvious work-arounds, and I'm not sure if I'd consider them all that bad: 1) use a caching http proxy for the downloads (in my experience cargo itself is pretty quick), or 2) use a shared user account for building ("sudo -s cargo-build; ... ").

I'm not sure concurrently writing to a group-writeable cache-folder is such a hot idea -- even things like apt use a write-lock when doing updates.

I suppose a third option is to mount (or probably symlink) ~/.cargo on a filesystem with deduplication -- but that wouldn't buy you caching for free (might work well coupled with a caching http proxy though).