Use workflow_run event to deploy pull request previews

.github/workflows/build-preview.yml

@@ -9,14 +9,21 @@ jobs:
   build-preview:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v1
-      - uses: actions/setup-node@v1
+      - uses: actions/checkout@v2
+      - uses: actions/setup-node@v2
         with:
-          node-version: 16
-          registry-url: https://registry.npmjs.org/
+          node-version: '16'
       - run: npm ci
       - run: npm run build-site
       - uses: actions/upload-artifact@v2
         with:
           name: site
           path: build/site
+      - name: Store pull request number for later use
+        run: |
+          mkdir -p build/pr
+          echo ${{github.event.number}} > build/pr/number
+      - uses: actions/upload-artifact@v2
+        with:
+          name: pr
+          path: build/pr