Fix GITHUB_TOKEN value in prepare-mergeback-branch

Author: mbg

.github/actions/prepare-mergeback-branch/action.yml

@@ -48,7 +48,7 @@ runs:
         BASE_BRANCH: "${{ inputs.base }}"
         HEAD_BRANCH: "${{ inputs.head }}"
         NEW_BRANCH: "${{ inputs.branch }}"
-        GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"
+        GITHUB_TOKEN: "${{ inputs.token }}"
       run: |
         set -exu
         pr_title="Mergeback ${VERSION} ${HEAD_BRANCH} into ${BASE_BRANCH}"