I see that deploy-rs is part of nixpkgs now, but the github repo still refers to itself for a flake, and it includes helper functions in the flake.
So how are you supposed to use it? Still use the flake for configuration but only run the nixpkgs binary?
It was also new to me that deploy-rs is in Nixpkgs.
I guess if you want to specify your own deployments, you still need to import the flake (is that so?). However, for the executable you can use the version in Nixpkgs! I guess the advantage of the Nixpkgs version is that it will be pre-built and more stable but updates may lag behind.
So what I do now is pretty ugly; I needed the library code for the activation, but it included a reference to the deploy-rs in the flake, so I copied that to my flake and changed it to use the nixpkgs deploy-rs instead.