Earlier quoted context omitted.
I do the same with Nix as it works for macOS builds as well It has the massive benefit of solving the lock-in problem. Your workflow is generally very short so it is easy to move to an alternative CI if (for example) Github were to jack up their prices for self hosted runners... That said, when using it in this way I personally love Github actions
Whata the killer benefit of nix over, like, a docker file or a package.lock or whatever?
Nix specifies dependencies declaritively, and more precisely, than Docker (does by default), so the resulting environment is reproducibly the same. It caches really well and doubles as a package manager.
Despite the initial learning curve, I now personally prefer Nix's declarative style to a Dockerfile