Another option: I often just manually vendor the nix files for the version of the package and/or service I want.

You can supply a particular package version to most services e.g. `services.nginx.package`.

Or if you need to vendor a service, you can copy it into your local repo and use `disabledModules` to prevent the loading of the original.

I haven't used them yet, but I think flakes will make this a lot easier, if my understanding of them is correct.