Update Zig toolchain

Hi! `pgzx` seems not compatible with the latest stable Zig release due to breaking changes, for example in [std.io](https://devclass.com/2025/07/07/zig-lead-makes-extremely-breaking-change-to-std-io-ahead-of-async-and-awaits-return) and [removal of usingnamespace](https://github.com/ziglang/zig/issues/20663). Not sure how much work it is to update but will have a quick look later. 