The main binary cache for NixOS is built using Hydra, an open-source CI system for Nix which is much older than Cachix.

Cachix is a proprietary SaaS alternative to Hydra's binary caching component. It's free to use for F/OSS, and it's much easier to set up than Hydra, so plugging Cachix in to a generic CI system like Travis or GitHub actions is a popular choice for open-source projects in the Nix community.