ci: fix PR not triggering workflows by switching back to pytestbot access token

The builtin actions token is not allowed to trigger further workflows.
So when we create a PR using that token, the PR checks don't run, and a
maintainer must close & open the PR to make them run. This is quite
annoying, so switch back to the "machine user" approach.

Fixes #9554.

.github/workflows/backport.yml

@@ -48,4 +48,4 @@ jobs:
           --title "${subject}" \
           --body "Backport of PR #${{ github.event.number }} to $target_branch branch. PR created by backport workflow."
       env:
-        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        GITHUB_TOKEN: ${{ secrets.CHATOPS }}