zig: avoid using vendored libc++ on macOS

[carlocab]

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Currently, zig still builds its own libc++. This causes issues like #278849.

We should report this upstream, but, as a temporary bandaid, since
mixing libc++ versions can cause all sorts of trouble, let's try a small
fix for now.

I used Claude to diagnose the problem and prepare a fix. I reviewed all changes made here.