Improve flake.nix, conforming to the output schema

[bryango]

Thank you for this wonderful program! This adds a few minor improvements for the flake.nix which makes it work better for myself.

• Point nixpkgs to the "unstable" channel which enjoys binary caches, and bump flake.lock
• Do not import nixpkgs; instead, use legacyPackages.${system} for better performance and composability.
• Abstract pinix into a "package" and invoke with pkgs.callPackage, for future convenience.
• Rewrite flake outputs such that it works well with the nix build and nix develop commands.

See: https://nixos.org/manual/nix/unstable/command-ref/new-cli/nix#installables