label-pull-requests action should avoid restoring labels removed by maintainers

[carlocab]

It's really annoying to keep fighting with the label-pull-requests action when you've removed, say, CI-linux-self-hosted (e.g. because you know the bottle cache will kick in) but it keeps restoring it after every push.

This may require checking who removed the label before deciding to re-add it. Here's a GraphQL query for that:

query ($owner: String!, $repo: String!, $pr: Int!) {
  repository(owner: $owner, name: $repo) {
    pullRequest(number: $pr) {
      timelineItems(itemTypes: [UNLABELED_EVENT], last: 100) {
        nodes {
          ... on UnlabeledEvent {
            actor {
              login
            }
            label {
              name
            }
          }
        }
      }
    }
  }
}

It should be enough to check that the label wasn't removed by github-actions. Happy for this to be done with a simpler approach, though.