fix(ci): Remove nix cache action as it barely works

It takes 11 minutes to cache everything on a otherwise 2 minute job.
Nix doesn't even pull from it most of the time.

.forgejo/workflows/update.yml

@@ -12,8 +12,6 @@ jobs:
         uses: actions/checkout@v4
       - name: Install nix
         uses: https://github.com/DeterminateSystems/nix-installer-action@main
-      - name: Setup Action cache for nix
-        uses: https://github.com/DeterminateSystems/magic-nix-cache-action@main
 
       - name: Update flake.lock
         id: update