Use multi-line script to resolve app user id without nested quotes

Store the gh api result in a variable and move app-slug into an env var, addressing PR review feedback about nested double-quotes in the run step.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.github/workflows/update-pdf-renderer.yml

@@ -51,7 +51,10 @@ jobs:
         if: steps.check.outputs.changed == 'true'
         env:
           GH_TOKEN: ${{ steps.app-token.outputs.token }}
-        run: echo "user-id=$(gh api "/users/${{ steps.app-token.outputs.app-slug }}[bot]" --jq .id)" >> "$GITHUB_OUTPUT"
+          APP_SLUG: ${{ steps.app-token.outputs.app-slug }}
+        run: |
+          user_id="$(gh api "/users/${APP_SLUG}[bot]" --jq .id)"
+          echo "user-id=${user_id}" >> "$GITHUB_OUTPUT"
 
       - name: Create Pull Request
         if: steps.check.outputs.changed == 'true'