workflow: add git fetch before rebasing

.gitea/workflows/update-flake.yaml

@@ -25,6 +25,7 @@ jobs:
       - name: Rebase from master branch
         shell: bash
         run: |
+            git fetch origin master
             commits_ahead=$(git rev-list --count HEAD..origin/master)
             echo "Commits ahead: $commits_ahead"
             git log --oneline -5