Try requiring members to sign off on commits too.

[ScottTodd]

Testing https://github.com/dcoapp/app workflows

• [x] Commit without sign off: https://github.com/iree-org/iree-experimental/pull/241/checks?check_run_id=24754686903 (failed as expected)
• [x] PR description sign-off https://github.com/iree-org/iree-experimental/pull/241/checks?check_run_id=24754763828 (would be nice if this worked, but it failed)
• [x] Apply suggested fix git rebase HEAD~2 --signoff && git push -f https://github.com/iree-org/iree-experimental/pull/241/checks?check_run_id=24754897593 (worked)
• [x] Accept suggestion on GitHub (worked, no need to edit the commit message)
• [x] Sign-off via CLI (worked, --signoff)
• [x] Sign-off via git-gui
• [x] Sign-off via VSCode