Add magic-nix-cache to flake update action

Author: jchv

.github/workflows/flake-update.yml

@@ -14,6 +14,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/determinate-nix-action@main
+      - uses: DeterminateSystems/magic-nix-cache-action@main
       - name: Update Nix flake inputs
         run: nix flake update
       - name: Run `nix build`