Add node modules cache, setup node 12

.github/workflows/build-release.yml

@@ -14,6 +14,24 @@ jobs:
       - name: Checkout code
         uses: actions/checkout@master
 
+      - name: Cache node modules
+        uses: actions/cache@v2
+        env:
+          cache-name: cache-node-modules
+        with:
+          # npm cache files are stored in `~/.npm` on Linux/macOS
+          path: ~/.npm
+          key: ${{ runner.os }}-build-${{ env.cache-name }}-${{ hashFiles('**/package-lock.json') }}
+          restore-keys: |
+            ${{ runner.os }}-build-${{ env.cache-name }}-
+            ${{ runner.os }}-build-
+            ${{ runner.os }}-
+
+      - name: Use Node.js 12.x
+        uses: actions/setup-node@v1
+        with:
+          node-version: 12.x
+
       - name: Build Gutenberg plugin ZIP file
         run: ./bin/build-plugin-zip.sh
         env: