Pin checkout action in release workflow

[AndrewMohawk]

Summary

• Pin the release workflow's checkout action to the current actions/checkout v4 commit SHA.

Security context

• This prepares the repo for enabling the GitHub Actions repository setting that requires actions to be pinned to full-length commit SHAs.
• Repo Actions settings have already been tightened to read-only default GITHUB_TOKEN permissions and selected GitHub-owned actions only.

Validation

• Verified the pinned SHA matches the current actions/checkout v4 tag target via git ls-remote.