feat: update all inputs, only PR if specified inputs updated

For updating my NixOS configuration, I would like to only update my flake input automatically on `nixpkgs/nixos-unstable` updates. However, when there is such an update, I would also like other inputs to be updated alongside it. 

Unsure if this is outside the scope of the action, and it is I think possible to do it manually, but would be nice to have it here instead.