chore!: make cli package/app compatible with flake

[Pegasust]

Basically make the cli derivation to conform with pkgs.lib.getBin "standard", so that flakes can recognize the binary

Before:

% ❯ nix run .#
error: unable to execute '/nix/store/8ich1m9piml9g77ik965ixnn27g6gxqq-poetry2nix-cli
-0/bin/poetry2nix-cli': No such file or directory

After:

% ❯ nix run .#
warning: Git tree '/Users/htran/local_repos/poetry2nix' is dirty
usage: .poetry2nix-cli-wrapped [-h] {lock} ...
.poetry2nix-cli-wrapped: error: the following arguments are required: subcommand