Add back in deploy message

Author: juliasilge

.github/workflows/bookdown.yaml

@@ -43,4 +43,4 @@ jobs:
           NETLIFY_SITE_ID: ${{ secrets.NETLIFY_SITE_ID }}
         run: |
           npm install netlify-cli -g
-          netlify deploy --prod --dir _book  
+          netlify deploy --prod --dir _book  --message "Deploy from GHA: ${{ github.event.pull_request.title || github.event.head_commit.message }} (${{ github.sha }})"