refactor: Add overlay.nix and pkgs dir, clean-up flake.nix, update all inputs (#22)

* refactor: Add overlay, do some cleanup

* Update nixpkgs flake input

* Run nix fmt on repo with new nixfmt-rfc-style

* Update all flake inputs

* check flake on all systems

Author: mitchmindtree

.github/workflows/ci.yml

@@ -25,7 +25,7 @@ jobs:
       - uses: cachix/install-nix-action@v16
         with:
           nix_path: nixpkgs=channel:nixos-unstable
-      - run: nix flake check --no-update-lock-file
+      - run: nix flake check --no-update-lock-file --all-systems
 
   # Checks the nix formatting.
   template-fmt-check: