Try reinstating cache

Author: microbit-matt-hillsdon

.github/workflows/node.js.yml

@@ -24,6 +24,7 @@ jobs:
         uses: actions/setup-node@v2
         with:
           node-version: ${{ matrix.node-version }}
+          cache: "npm"
       - run: npm ci
       - run: npm run build --if-present
       - run: npm test