ci: drop magic-nix-cache-action since it's failing

.github/workflows/ci.yaml

@@ -91,13 +91,6 @@ jobs:
         with:
           github_access_token: ${{ secrets.GITHUB_TOKEN }}
 
-      - name: Cache nix store
-        uses: DeterminateSystems/magic-nix-cache-action@b46e247b898aa56e6d2d2e728dc6df6c84fdb738
-        with:
-          # I do NOT consent to share build cache between FlakeHub.
-          # This should NOT be the default.
-          use-flakehub: false
-
       - name: Flake check
         run: nix flake check --no-update-lock-file --show-trace