Backport workflow: try using the app token

GITHUB_TOKEN is no longer defined; we should use either the workflow token or the app one. Here we try using the app one.

Author: smowton

.github/workflows/update-release-branch.yml

@@ -134,7 +134,7 @@ jobs:
         echo SOURCE_BRANCH=${SOURCE_BRANCH}
         echo TARGET_BRANCH=${TARGET_BRANCH}
         python .github/update-release-branch.py \
-          --github-token ${GITHUB_TOKEN} \
+          --github-token ${{ steps.app-token.outputs.token }} \
           --repository-nwo ${{ github.repository }} \
           --source-branch ${SOURCE_BRANCH} \
           --target-branch ${TARGET_BRANCH} \