Update all action refs to use pinned commit SHAs instead of floating tags:
- actions/checkout: @v6 → @8e8c483 (v4)
- actions/setup-go: @v6 → @0c52d54 (v5)
- actions/upload-artifact: @v6 → @65d8626 (v4)
Pinned SHAs improve reproducibility and reduce supply chain risk by
preventing accidental or malicious changes in action releases. Aligns
with repository conventions used in other workflows (e.g., go.yml).
01da26fbcb