Hmm, shell a package from N and then from nixpkgs and see if it downloads anything? They have to be the same reference though - I see that the default nixpkgs is nixpkgs-unstable not nixos-unstable (I changed it above now).
yeah, you can use it like nix shell nixpkgs#s3cmd or nix shell flake:nixpkgs#s3cmd. I assumed your post was about renaming nixpkgs to something shorter to save some keystrokes haha
No, it is, see original post now. I was pointing N to github:NixOS/nixpkgs/nixpkgs-unstable and then realized I can just point N to flake:nixpkgs and it works fine!