ci: skip Go cache save on Windows to stop flaky cache-save post-step

[joshua-temple]

What this change does

Disables the Go module/build cache on windows-latest runners across the matrix workflows (sink, source, test, release) by gating setup-go's cache: cache: ${{ runner.os != 'Windows' }}. Linux and macOS keep caching unchanged.

Why

The Post Run actions/setup-go cache-save step flakes intermittently on windows-latest: the tar/zstd cache.tzst write fails and takes the whole job down with it even when the test step passed (seen on PR #132, sink unit). setup-go has no "restore-but-don't-save" option and a packaged action's post-step can't be made non-fatal, so gating the cache off on Windows is the deterministic fix. The cost is a cache miss on the Windows legs (a little slower, modules re-downloaded); the benefit is no more spurious red on green PRs.

Relates to: the spurious failure on #132.

Checklist

• Commits are signed off (git commit -s) per the DCO
• Conventional commit messages (type: subject)
• YAML validated; change is CI-only (no code/tests affected)
• mage check unaffected (workflow-only change)