Fix CLA issues in workflows (#6108)

Per [b/353295303](https://b.corp.google.com/issues/353295303),

This updates our `post_release_cleanup` workflow to use the
`google-oss-bot` as the signee. This should allow the workflow to pass
CLA, as `github-actions` is not supported.

Note that while `create_release` _does_ create a PR that could take
advantage of this, we've chosen to keep it in a state where the CLA
fails; such that the PR is not accidentally merged (as _that_ PR should
_not_ be merged).

Author: daymxn

.github/workflows/post_release_cleanup.yml

@@ -28,6 +28,9 @@ jobs:
       - name: Create Pull Request
         uses: peter-evans/create-pull-request@v4
         with:
+          token: ${{ secrets.GOOGLE_OSS_BOT_TOKEN }}
+          committer: google-oss-bot <[email protected]>
+          assignees: ${{ github.actor }}
           base: 'main'
           branch: 'releases/${{ inputs.name }}.mergeback'
           add-paths: |