* ci: migrate remaining pinned actions to node24
actions/cache@v4 and mlugg/setup-zig@v2.2.1 still target Node 20, which
GitHub removes from runners on 2026-09-23. Bump actions/cache to v6.1.0 and
replace mlugg/setup-zig with vercel-labs/setup-zig@v1.0.2.
vercel-labs/setup-zig takes only `version`, so the use-cache/cache-size-limit
tuning and Zig global-cache reuse are dropped. See the PR for alternatives.
* ci: restore zig caching for vercel setup-zig
vercel-labs/setup-zig has no cache controls, so redirect the Zig global cache
into the workspace .zig-cache and run the Zig build cache step on Linux too.
Drop this commit if we switch to a setup action that owns Zig caching, e.g.
step-security/setup-zig or xyzzylabs/setup-zig.
JJ Liebig
Can Celik