CI: remove deprecated magic-nix-cache-action

Author: fufexan

.github/workflows/nix.yml

@@ -8,7 +8,6 @@ jobs:
     - uses: actions/checkout@v4
 
     - uses: DeterminateSystems/nix-installer-action@main
-    - uses: DeterminateSystems/magic-nix-cache-action@main
 
     # not needed (yet)
     # - uses: cachix/cachix-action@v12