Nix: allow building swww from the source directly

[JohnRTitor]

This PR adds support for building nix binaries directly from this repo.

NixOS user's can easily add this repo to their flake.nix and use the latest version directly from this repository.

```nix inputs.swww.url = "github:LGFae/swww"; ``` Then in `configuration.nix` ```nix environment.systemPackages = [ inputs.swww.packages.${pkgs.system}.swww ]; ```

Build instructions for nix after cloning this repo:

With `nix-build`: ```bash nix-build ``` With `nix build` (flakes): ```bash nix build .#swww ```

Why?

Motivation for this comes from the need to have a swww-git package on NixOS to get the latest changes. However nixpkgs is inefficient for hosting a -git package, since approval and merging of PRs do take a lot of time. Adventurous users can use the flake directly to stay upto date on the latest bugfixes and features.

A lot of packages already utilise this to provide nix packages directly from their source repository.

To-Do

• [x] README docs need to be updated.
• [x] Implement a workflow that'll automatically update the dependencies

[LGFae]

I'll be honest, I don't like having files I am not entirely familiar with in the repo. But I remember someone asked this before, and these seem fairly straightforward, so I will be merging them.

Thanks!

[JohnRTitor]

Thanks. These changes are pretty low-maintainance, only flake.lock needs to be updated (there is a automatic GitHub action that'll run every Sunday), unless you add a new dependency to the source.

I am happy to help with further maintanence/support related to this. Just @ me.