ci: define and enforce the Zig toolchain policy - #16

Process completed with exit code 100.

Failed to save: Unable to reserve cache with key pantry-v2-0.11.64-linux-x64-7ae3236813f334d1c8d370bf1a5c967fe9d7697672f793fbcc153ec23e54f6e7-none-zig@0.17.0-dev.2163+89ff10d56, another job may be creating this cache.