Fix: add checkout to workflow-variables job in promote-branch.yml

Agent-Logs-Url: https://github.com/petesramek/polyline-algorithm-csharp/sessions/006faa11-c1a2-4108-bef1-1b5bfa33e040

Co-authored-by: petesramek <2333452+petesramek@users.noreply.github.com>

Author: Copilot

.github/workflows/promote-branch.yml

@@ -60,6 +60,8 @@ jobs:
         target-branch-exists: ${{ steps.check-target-branch-exists.outputs.target-branch-exists }}
         pull-request-exists: ${{ steps.check-pull-request-exists.outputs.pull-request-exists }}
     steps:
+      - name: 'Checkout ${{ github.head_ref || github.ref }}'
+        uses: actions/checkout@v6
       - name: 'Set target branch'
         id: set-target-branch
         run: |