Fix Check Nix flake CI failures

[scottisloud]

Summary

• Update flake.lock to current nixpkgs-unstable (June 16) so the flake-checker 30-day freshness check passes
• Disable FlakeHub in magic-nix-cache-action to match the update-flake-dependencies workflow and avoid auth noise in logs

Test plan

• Check Nix flake workflow passes on this PR
• nix flake check succeeds locally (optional)

[github-actions]

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