ci: add Cachix caching for Nix builds

.github/workflows/ci.yml

@@ -18,6 +18,12 @@ jobs:
         with:
           nix_path: nixpkgs=channel:nixos-unstable
 
+      - name: Setup Cachix
+        uses: cachix/cachix-action@v15
+        with:
+          name: jellarr
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
+
       - name: Install Node.js dependencies
         run: |
           nix develop --command pnpm install