fix: remove magic-nix-cache-action causing platform mapping error

Removed DeterminateSystems/magic-nix-cache-action to resolve "Cannot convert ArchOs (amd64-linux)" error in Gitea Actions. The action is GitHub-specific and doesn't work with act_runner's platform detection. Workflow will now use default cache.nixos.org substituter.

Note: Consider migrating to Attic binary cache for better performance in the future.

.gitea/workflows/nix-update.yml

@@ -67,9 +67,6 @@ jobs:
             echo "has_flake=false" >> $GITHUB_OUTPUT
           fi
 
-      - name: Setup Nix Binary Cache
-        uses: DeterminateSystems/magic-nix-cache-action@main
-
       - name: Update Packages
         id: update
         run: |