You can use both Nix and Cargo at the same time, and they accomplish different things. Nix's purpose is more like rustup, except it works for each program on your computer

Cargo does not attempt to solve the same problems as Nix (if you depend on any software not written in Rust). A checked in Nix shell is genuinely great documentation on the expected system environment.

Can Cargo install deployment tools like Ansible? I find myself using nix for those kind of tools despite how good $lang package manager is.

Using nix to install Ansible, oof you're hurting me..

Cargo isn't intended for that. If there was something like Ansible implemented in Rust, then you could use Cargo to install it.