ci: cache npm dependencies on CI

.github/workflows/ci.yml

@@ -10,6 +10,14 @@ jobs:
       - uses: actions/setup-node@v1
         with:
           node-version: '12.x'
+      - uses: actions/cache@v2
+        with:
+          path: |
+            ~/.npm
+            **/node_modules
+          key: ${{ runner.os }}-node-${{ hashFiles('**/package-lock.json') }}
+          restore-keys: |
+            ${{ runner.os }}-node-
       - run: npm install
       - run: npm test -- --ci --coverage
   release: