fix: signoff commit

[ramonpetgrave64]

Followup to https://github.com/slsa-framework/slsa-verifier/pull/760

Fix the .github/workflows/update-actions-dist-post-commit.yml workflow to also signoff commit

Testing

• [x] Invoked this PR's branch copy of the workflow against #717, and it did signoff the commit.
  • https://github.com/slsa-framework/slsa-verifier/pull/717/commits/9670f76ab8b64299c58096a19212b558dce94b90

[ramonpetgrave64]

@ianlewis