ci: fix backport of workflow failures

Apparently passing a token with sufficient permission to the action
isn't enough, we also need to check out using that token.
See korthout/backport-action#368

Signed-off-by: Paul Meyer <49727155+katexochen@users.noreply.github.com>

.github/workflows/backport.yml

@@ -23,6 +23,8 @@ jobs:
       pull-requests: write
     steps:
       - uses: actions/checkout@44c2b7a8a4ea60a981eaca3cf939b5f4305c123b # v4.1.5
+        with:
+          token: ${{ secrets.NUNKI_CI_COMMIT_PUSH_PR }}
       - name: Create backport PRs
         uses: korthout/backport-action@ef20d86abccbac3ee3a73cb2efbdc06344c390e5 # v2.5.0
         with: