ci: use bot token for backport

This allows GitHub Action workflows to be triggered for the PR created
by the bot.

As things are now triggered by the bot, elevated permissions of the
workflow itself are no longer required.

Signed-off-by: Hidde Beydals <hidde@hhh.computer>
(cherry picked from commit 80eeff2739)

.github/workflows/backport.yaml

@@ -10,9 +10,6 @@ permissions:
 jobs:
   pull-request:
     runs-on: ubuntu-latest
-    permissions:
-      contents: write
-      pull-requests: write
     if: github.event.pull_request.state == 'closed' && github.event.pull_request.merged && (github.event_name != 'labeled' || startsWith('backport:', github.event.label.name))
     steps:
       - name: Checkout
@@ -23,6 +20,8 @@ jobs:
         uses: korthout/backport-action@bf5fdd624b35f95d5b85991a728bd5744e8c6cf2 # v1.3.1
         # xref: https://github.com/korthout/backport-action#inputs
         with:
+          # Use token to allow workflows to be triggered for the created PR
+          github_token: ${{ secrets.BOT_GITHUB_TOKEN }}
           # Match labels with a pattern `backport:<target-branch>`
           label_pattern: '^backport:([^ ]+)$'
           # A bit shorter pull-request title than the default