Nix doesn't sandbox builds by default on macOS. You can try enabling it yourself with `sandbox = true` in nix.conf, but Things May Break.

The Nix sandbox is also not really meant as a security boundary; there's no effort put into preventing sandbox escapes, and lots of stuff leaks from the host into the sandbox environment. You really want something like gVisor or a full VM if you want to build untrusted packages.