As I understand it, Zig is trying to bring in almost all the stuff that would usually be done by external tools. Zig has its own solution for finding system libraries (instead of pkg-config), it integrates its own C/C++ compiler, and you can do code generation with comptime (or, worst-case, a Zig program that can also be compiled to WebAssembly) instead of an external script. So I think there's a good chance that you'll be able to build most Zig projects entirely inside the sandbox someday.

There's still the obvious problem that if the build system emits malicious code, you'll probably run that code anyway. Personally I think this kind of sandboxing is more useful for enforcing build reproducibility rather than, like, protecting you from viruses in the build.zig file.