Zig version

Mach uses nominated Zig versions where we nominate a specific nightly Zig version to use every 6 weeks. This allows us to target a nightly version of Zig, while still allowing an ecosystem of Zig packages to be compatible with each-other. We build and test against the latest nominated version, so other versions like stable/nightly Zig are usually incompatible.

We encourage using Mach’s main branch rather than a tagged release) for now.