Upload the branch version of the website for pull requests

Also change the main upload action to only execute when the action is
not triggered by a PR.

.github/workflows/build_and_deploy.yaml

@@ -25,15 +25,24 @@ jobs:
       - name: Build the website
         run: ./hugo
 
-      - name: Set dry run
-        run: echo "DRY_RUN=${{ github.event_name == 'pull_request' && 'true' || 'false' }}" >> $GITHUB_ENV
+      - name: Upload the branch version of the website
+        uses: Dylan700/sftp-upload-action@latest
+        with:
+          username: webstage
+          server: web.ivoa.net
+          dry-run: ${{ env.DRY_RUN }}
+          key: ${{ secrets.STAGE_ID }}
+          uploads: |
+            ./public/ => ./ivoa-web-stage/v/${{ github.ref_name }}
+          delete: 'true'
+        if: github.event_name == 'pull_request'
 
       - name: Deploy the website
         uses: Dylan700/sftp-upload-action@latest
         with:
           username: webstage
           server: web.ivoa.net
-          dry-run: ${{ env.DRY_RUN }}
           key: ${{ secrets.STAGE_ID }}
           uploads: |
             ./public/ => ./ivoa-web-stage/
+        if: github.event_name != 'pull_request'