Using Github App to trigger CI on backport PRs (opensearch-project#175)

Signed-off-by: Vacha Shah <vachshah@amazon.com>

Author: VachaShah

.github/workflows/backport.yml

@@ -13,8 +13,16 @@ jobs:
       pull-requests: write
     name: Backport
     steps:
+      - name: GitHub App token
+        id: github_app_token
+        uses: tibdex/github-app-token@v1.5.0
+        with:
+          app_id: ${{ secrets.APP_ID }}
+          private_key: ${{ secrets.APP_PRIVATE_KEY }}
+          installation_id: 22958780
+
       - name: Backport
         uses: VachaShah/backport@v1.1.4
         with:
-          github_token: ${{ secrets.GITHUB_TOKEN }}
+          github_token: ${{ steps.github_app_token.outputs.token }}
           branch_name: backport/backport-${{ github.event.number }}