garnix is a smol and free CI service for Nix that works really well for open source projects. You just add a flake.nix to your project and it starts building and caching your project. Did I mention that it's free (so far)?
The thing I love most about it is the minimal UX. I know that sort of thing is not for everyone, but I love that to death. I'm a big fan of "tacit" tools with very few options and minimal logging (if any).