ci: add a rebase checkbox to onboarding PRs

.gitea/workflows/renovate.yaml

@@ -36,6 +36,8 @@ jobs:
           # names. Ticking a box Renovate wrote dispatches such a run (edb-infra, roles/gitea.md).
           RENOVATE_AUTODISCOVER: "true"
           RENOVATE_AUTODISCOVER_FILTER: ${{ inputs.repository || 'edb/*' }}
+          # A rebase box on onboarding PRs too, so they can be fast-forwarded. Experimental.
+          RENOVATE_ONBOARDING_REBASE_CHECKBOX: "true"
           # Raises the GitHub API rate limit for release notes and github-tags lookups
           GITHUB_COM_TOKEN: ${{ secrets.RENOVATE_GITHUB_COM_TOKEN }}
           LOG_LEVEL: info