build: reduce cold-cache build times

[emidoots]

Currently due to the way our build.zig scripts work, and due to how the Zig package manager works, this command in the mach-core repository:

rm -rf zig-cache zig-out/ ~/.cache/zig && zig build

Results in two clones of https://github.com/hexops/xcode-frameworks - and a decent amount of work (I believe?) spend zig building our dependencies which doesn't actually end up being used in the final binary you're actually trying to build. We should be able to reduce this.

[emidoots]

Helped by https://github.com/hexops/mach/issues/903

Being tracked historically at https://wrench.machengine.org/stats/