Avoid comment error on bot PRs

For PRs opened by dependabot or other reduced privelegde bots, this workflow can't run https://github.com/holochain/actions/blob/main/.github/workflows/changelog-preview-comment.yml. I'd prefer to avoid marking the CI failed on my browser tabs when I'm monitoring multiple builds.

Start by reading .github/workflows/changelog-preview-comment.yml and inspect how it behaves for pull requests opened by dependabot or other reduced-privilege bots. Verify the workflow’s failure behavior on such PRs, then confirm that those builds no longer appear failed when the comment action cannot run.

Written by the indexing model from the issue text.