Give preview jobs higher permissions so that preview comments can work

[holly-cummins]

@maxandersen spotted that his recent blog post didn't get a preview. On investigation, it did get a preview, but it didn't get a comment on the PR with the preview link. On further investigation, previews have been failing for 3 weeks.

Based on the timing and content, I think the problem must have been caused by #2042. It contains the fatal equivalent of // this should never happen, this should be enough to ... :)

    permissions:
      actions: read
      # this should be enough to maintain comments as commenting on pull requests is done through an issues API
      issues: write
      pull-requests: read

The GitHub workflow changes can only be tested by YOLO-ing them in and then hoping, so regressions are super-easily done. I'm hoping this change fixes it, but, you know ... it can only be tested by YOLO-ing it in.

[github-actions]

🙈 The PR is closed and the preview is expired.