ci: update node version

Author: ark120202

.github/workflows/ci.yml

@@ -8,7 +8,10 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v1
-      - uses: actions/setup-node@v1
+      - uses: actions/checkout@v2
+      - name: Use Node.js 12
+        uses: actions/setup-node@v1
+        with:
+          node-version: 12
       - run: yarn --frozen-lockfile
       - run: yarn lint