Switch to punica-sgmv kernel from the Hub (#3236)
* Switch to punica-sgmv kernel from the Hub
This also switches (temporarily) to the tgi-nix/kernel-builder merge
branch, bumping up to CUDA 12.8 (same as non-Nix Torch).
* nix: client depends on aiohttp
This probably worked before the nixpkgs bump because a dependency
propagated aiohttp.