fix: automerge with automatic token

[davdhacs]

• Give write access to the automatic action github token.
• Use the automatic token in the action.

The automerge action needs a write-allow github token. Triggered by pull_request_target the action is not given the dependabot secrets:

For workflows initiated by Dependabot (github.actor == 'dependabot[bot]') using the pull_request_target event, if the base ref of the pull request was created by Dependabot (github.actor == 'dependabot[bot]'), the GITHUB_TOKEN will be read-only and secrets are not available.
https://docs.github.com/en/code-security/dependabot/working-with-dependabot/automating-dependabot-with-github-actions#responding-to-events

[davdhacs]

closed in favor of #125