Revert to Node v8, newer versions can't build

.github/workflows/ci.yml

@@ -10,7 +10,7 @@ jobs:
       - name: Setup Node.js
         uses: actions/setup-node@v1
         with:
-          node-version: 12.x
+          node-version: 8.x
       - name: Install dependencies
         run: yarn install --frozen-lockfile --ignore-scripts
         working-directory: website