ci(nix): use setup-nix action for flake update workflow

Author: ryoppippi

.github/workflows/nix-flake-update.yaml

@@ -17,8 +17,8 @@ jobs:
       - name: Checkout repository
         uses: actions/checkout@v4
 
-      - name: Install Nix
-        uses: DeterminateSystems/nix-installer-action@main
+      - name: Setup Nix
+        uses: ./.github/actions/setup-nix
 
       - name: Update flake.lock
         run: nix flake update