#GitHub down? Nixpkgs can and should in most cases be fetched from channels.nixos.org because that is redistributed world-wide via #fastly with much higher availability and IPv6 support.
E.g. in your #flake use
nixpkgs.url = "https://channels.nixos.org/nixos-26.05-small/nixexprs.tar.zst";
instead of the github-flavored expression. Works for all supported channels.
