zig: Downgrade to 0.13 with local LLVM

[GZGavinZhao]

Summary

Bring back Zig from 0.14 snapshot to stable 0.13 release using in-tree LLVM build.

Test Plan

• Built ghostty
• Zig x86 test suite all passed.

Checklist

• Package was built and tested against unstable
• This change could gainfully be listed in the weekly sync notes once merged

[GZGavinZhao]

Last rebuild to see if I can get rid of networking: true.

[GZGavinZhao]

Admittedly it's a little ugly with the export CC and export CMAKE_C_COMPILER_LAUNCHER. We can get rid of that if we don't use clang: yes.