CI: test on same nodejs version as using vercel

Author: f3l1x

.github/workflows/workflow.yml

@@ -12,7 +12,7 @@ jobs:
       - name: Setup Node
         uses: actions/setup-node@v1
         with:
-          node-version: '13.x'
+          node-version: '12.x'
 
       - name: Dependencies
         run: |