[PETOSS-829] Use var for app_id as per docs

Author: the-chris-mitchell

.github/workflows/pr-validation.yml

@@ -36,7 +36,7 @@ jobs:
         id: get_access_token
         uses: actions/create-github-app-token@v2
         with:
-          app-id: ${{ secrets.XERO_GITHUB_APP_ID }}
+          app-id: ${{ vars.XERO_CODEGEN_APP_ID }}
           private-key: ${{ secrets.XERO_GITHUB_APPLICATION_KEY }}
           owner: xero-internal
           repositories: xeroapi-sdk-codegen