Upgrade `flake.nix` to produce a package with the binary

[RCasatta]

At the moment the flake.nix is used only to create the dev environment, it would be more useful to produce also the peerswap binary in a package so that can be properly referenced in a nix env

[YusukeShimizu]

Thank you for your suggestion.
I believe the priority is low unless specific requirements or requests arise, but we will consider addressing it.