ci: disable flakehub in nix magic cache

Add configuration to disable FlakeHub in the Magic Nix Cache action
for the nix-flake-test job, ensuring reliable Nix dependency caching.

Author: MBanucu

.github/workflows/ci.yml

@@ -74,6 +74,8 @@ jobs:
         uses: DeterminateSystems/determinate-nix-action@v3
       - name: Enable Magic Nix Cache
         uses: DeterminateSystems/magic-nix-cache-action@v13
+        with:
+          use-flakehub: false
       - name: Cache Bun dependencies
         uses: actions/cache@v5
         with: