use node 12 on ci (#966)

.github/workflows/validate.yml

@@ -7,10 +7,10 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v1
-      - name: Use Node.js 10.x
+      - name: Use Node.js 12.x
         uses: actions/setup-node@v1
         with:
-          node-version: 10.x
+          node-version: 12.x
       - name: npm install, lint, build, and test
         run: |
           npm install