workflows/backport: avoid retriggering workflows after adding "has: port to stable" label

After creating the backport successfully, we previously created the
"has: port to stable" label with the Nixpkgs CI App's token. This would
trigger another labeled event for the backport workflow. This only
appears as "skipped", so doesn't use any resources, but it clutters the
GitHub Actions output with useless skipped workflows.

Using `github.token` does not trigger any other workflows so avoids that
problem.

(cherry picked from commit 2566f9dcb4)

.github/workflows/backport.yml

@@ -9,7 +9,9 @@ on:
   pull_request_target:
     types: [closed, labeled]
 
-permissions: {}
+permissions:
+  contents: read
+  pull-requests: write
 
 jobs:
   backport:
@@ -48,7 +50,8 @@ jobs:
       - name: "Add 'has: port to stable' label"
         if: steps.backport.outputs.created_pull_numbers != ''
         env:
-          GH_TOKEN: ${{ steps.app-token.outputs.token }}
+          # Not the app on purpose to avoid triggering another workflow run after adding this label
+          GH_TOKEN: ${{ github.token }}
           REPOSITORY: ${{ github.repository }}
           NUMBER: ${{ github.event.number }}
         run: |