CI: pin the checkout action in official_release.yml to a commit
Every other workflow has pinned its external actions to full commit SHAs since 463f5a6ee122. `official_release.yml` was added after that and still uses the mutable `v4` tag, in the one job that runs with `contents: write`. This switches it to the same v6.0.2 commit that stockfish.yml already uses, with the same inputs.
I can't dry-run the release workflow from a fork, so this is validated as YAML only.
closes https://github.com/official-stockfish/Stockfish/pull/7253
No functional change