ci: fix manifest-PR push rejection on workflow re-run

[Copilot]

The call-manifest-pr-action job fails when the workflow is re-run on a PR opened event because nrfconnect/action-manifest-pr@main attempts a non-force push of auto-manifest-sdk-zephyr-<PR> to nordicbuilder/sdk-nrf, which is rejected if the branch already exists from a prior run. The synchronize handler uses -f; the opened handler does not.

Changes

• .github/workflows/manifest-PR.yml: Add a pre-step (gated on opened events) that deletes the existing remote branch via the GitHub API before the action runs. Uses || true so it is a no-op when the branch does not exist.

- name: Delete existing manifest branch if present
  if: github.event.action == 'opened'
  run: |
    PR_NUMBER="${{ github.event.pull_request.number }}"
    REPO_NAME="${{ github.event.repository.name }}"
    BRANCH="auto-manifest-${REPO_NAME}-${PR_NUMBER}"
    gh api --method DELETE "repos/nordicbuilder/sdk-nrf/git/refs/heads/${BRANCH}" 2>/dev/null || true
  env:
    GH_TOKEN: ${{ secrets.NCS_GITHUB_TOKEN }}