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.
Language-specific build facilities, like Cargo's build.rs and Zig's build scripts, typically have a limited scope - generating a bit of source code, discovering some linker flags, stuff like that. These scripts need to be run by LSP servers when opening the project in an editor to get basic features working, so that's a fairly risky thing.
They are also currently doing things like invoking CMake and other build systems, but you could definitely conceive of a world where that was a separate step in the build process, and that world seems pretty attractive to me.
A common pattern in Rust projects is to have a `*-sys` crate representing the C FFI bindings, and they typically also do something like invoke CMake or similar to actually build the C/C++ library underneath. But if you have a larger project that already integrates multiple build systems, this is really quite inconvenient in most cases.