cache node_modules on GH actions

Signed-off-by: Jari Kolehmainen <jari.kolehmainen@gmail.com>

.github/workflows/test.yml

@@ -20,6 +20,12 @@ jobs:
         with:
           node-version: ${{ matrix.node-version }}
 
+      - name: Cache node_modules
+        uses: actions/cache@v2
+        with:
+          path: '**/node_modules'
+          key: ${{ runner.os }}-modules-${{ hashFiles('**/yarn.lock') }}
+
       - run: make node_modules
         name: Install dependencies