ci: pin node version to 12.x

.github/workflows/frontend.yml

@@ -11,15 +11,11 @@ on:
 jobs:
   build:
     runs-on: ubuntu-16.04
-    strategy:
-      matrix:
-        node-version: [8.x, 10.x, 12.x]
     steps:
       - uses: actions/checkout@v1
-      - name: Use Node.js ${{ matrix.node-version }}
-        uses: actions/setup-node@v1
+      - uses: actions/setup-node@v1
         with:
-          node-version: ${{ matrix.node-version }}
+          node-version: '12.x'
       - name: yarn install, check and build
         run: |
           cd html