Execute automerges using a PAT

[brrygrdn]

Ironically, this repository itself runs into the issue whereby API calls performed by dependabot[bot] inside Actions can't trigger downstream workflows so our auto merge commits don't trigger CI currently.

This PR follows our own advice on how to workaround the problem with merges. I'll update the README once I've verified this works as intended.

[mwaddell]

@brrygrdn It looks like the PAT that's being used doesn't have the correct permissions set. See https://github.com/dependabot/fetch-metadata/runs/5359266073?check_suite_focus=true

gh pr merge --auto --merge "$PR_URL"
  shell: /usr/bin/bash -e {0}
  env:
    PR_URL: https://github.com/dependabot/fetch-metadata/pull/167
    GITHUB_TOKEN: ***
Message: Your token has not been granted the required scopes to execute this query. The 'enablePullRequestAutoMerge' field requires one of the following scopes: ['public_repo'], but your token has only been granted the: [''] scopes. Please modify your token's scopes at: https://github.com/settings/tokens., Locations: [{Line:1 Column:[7](https://github.com/dependabot/fetch-metadata/runs/5359266073?check_suite_focus=true#step:4:7)2}]
Error: Process completed with exit code 1.