Change backport bot token

[pmalek-sumo]

Description

Change from GITHUB_TOKEN to a personal access token because of this

The built-in secrets.GITHUB_TOKEN cannot yet be used because of this limitation:
https://github.community/t5/GitHub-Actions/Triggering-a-new-workflow-from-another-workflow/td-p/31676
In the meantime, use a token granting write access on the repo:

• a GitHub App token
  See https://github.com/marketplace/actions/github-app-token.
• a personal access token
  See https://help.github.com/en/articles/creating-a-personal-access-token-for-the-command-line.