Make auto-update run as the app

Signed-off-by: Matty Widdop <18513864+MattyTheHacker@users.noreply.github.com>

Author: MattyTheHacker

.github/workflows/pr-auto-update.yml

@@ -10,9 +10,15 @@ jobs:
       pull-requests: write
       contents: write
     steps:
+      - uses: actions/create-github-app-token@v2
+        id: app-token
+        with:
+          app-id: 1363586
+          private-key: ${{ secrets.PR_AUTO_UPDATE_PRIVATE_KEY }}
+
       - uses: CSSUoB/pr-auto-updater@v2.1.2
         env:
-          GITHUB_TOKEN: '${{ secrets.PR_AUTO_UPDATE_TOKEN }}'
+          GITHUB_TOKEN: ${{ steps.generate-token.outputs.token }}
           PR_FILTER: 'labelled'
           PR_LABELS: 'sync'
           MERGE_CONFLICT_ACTION: 'label'