Snappier tarball fetches with Nix

My recent deep dives into libgit2 have been pretty fruitful when it comes to finding new and unexpected bugs peculiarities of the library and the way nix holds it. So here goes: Nix stores unpacked tarballs fetched via builtins.fetchTarball and tarball-style flake inputs: so all github:/gitlab: and nixpkgs channel tarballs (like https://channels.nixos.org/nixos-unstable/nixexprs.tar.zst) in a bare git repository in $XDG_CACHE_HOME/nix/tarball-cache-v2. Unpacking each tarball now yields at most ...