feat: add cross-run binary caching to speed up repeated workflow runs

Use @actions/cache to persist the downloaded auths binary across workflow
runs. On GitHub-hosted runners each run gets a fresh VM, so without this
the binary is re-downloaded every time. Cache key is derived from the
download URL hash, so version changes automatically invalidate the cache.
Only pinned versions are cached; "latest" always re-downloads.

Author: bordumb