fix: add explicit read permissions to scheduled workflow

Agent-Logs-Url: https://github.com/geocompx/geocompr/sessions/19f46a52-d387-423d-b4a4-554ec62aada0

Co-authored-by: Robinlovelace <1825120+Robinlovelace@users.noreply.github.com>

Author: Copilot

.github/workflows/scheduled.yaml

@@ -7,6 +7,8 @@ jobs:
   bookdown:
     name: Render-Book
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
     container: ghcr.io/geocompx/suggests:latest
     env:
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}