zig build: use CacheMode.whole for build_runner.zig
Maybe after we have incremental compilation metadata serialization
and non-LLVM backends, it will make sense to switch this back. For now,
however, this makes successive `zig build` commands much faster.
1387d2f5acAndrew Kelley committed on 5/1/2022, 10:43:52 PMparent82b96ca