workflows/backport: persist credentials

This was added when introducing zizmor. It appears that the backport
action actually needs these credentials to persist, to be able to push
the branch.

.github/workflows/backport.yml

@@ -40,7 +40,7 @@ jobs:
         with:
           ref: ${{ github.event.pull_request.head.sha }}
           token: ${{ steps.app-token.outputs.token }}
-          persist-credentials: false
+          persist-credentials: true
 
       - name: Log current API rate limits
         env: