ci: use GitHub App for automatic-rebase (#11)

Signed-off-by: Kenji Miyake <[email protected]>

Author: kenji-miyake

.github/workflows/automatic-rebase.yaml

@@ -10,13 +10,19 @@ jobs:
     if: ${{ github.event.issue.pull_request != '' && contains(github.event.comment.body, '/rebase') }}
     runs-on: ubuntu-latest
     steps:
+      - name: Generate token
+        id: generate-token
+        uses: tibdex/github-app-token@v1
+        with:
+          app_id: ${{ secrets.APP_ID }}
+          private_key: ${{ secrets.PRIVATE_KEY }}
+
       - name: Checkout repository
         uses: actions/checkout@v2
         with:
-          token: ${{ secrets.GITHUB_TOKEN }}
           fetch-depth: 0
 
       - name: Automatic rebase
         uses: cirrus-actions/[email protected]
         env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GITHUB_TOKEN: ${{ steps.generate-token.outputs.token }}