PRs opened by draft-release workflow don't automatically trigger GHA builds

:clipboard: **Description**

An unexpected side effect of migrating the test builds to GHA is that workflows don't trigger on a PR that was submitted by a Github action (there are good reasons for this). Even though the PR is just editing markdown, we do need the build to pass because of the branch protection rules.

As a workaround, the actions will trigger if a human interacts with the PR. I generally do push another commit to clean up the auto-generated changelog anyway, so this isn't actually a huge deal.

I need to read https://github.com/peter-evans/create-pull-request/blob/main/docs/concepts-guidelines.md#triggering-further-workflow-runs and work out whether we just live with this or implement one of those solutions. They all come with tradeoffs and it might be that it is actually best to just stick with the current arrangement.

Read the linked create-pull-request guidance, then inspect the draft-release workflow that opens the PR. Determine whether to retain the current human-interaction workaround or implement one of the documented approaches, accounting for the branch protection requirement that builds pass; done means the decision and resulting behavior are clear.

Written by the indexing model from the issue text.