CI: fix workflow action deprecation

Author: f-f

.github/workflows/nix-flake.yml

@@ -18,7 +18,7 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - uses: DeterminateSystems/nix-installer-action@main
-      - uses: DeterminateSystems/magic-nix-cache-action@main
+      - uses: DeterminateSystems/flakehub-cache-action@main
       - name: "Check `nix develop` shell can open"
         run: nix develop --command "echo"
       # this Flake has no build target