CI: make use of the actions/setup-node's cache option

Author: XhmikosR

.github/workflows/ci.yml

@@ -19,6 +19,7 @@ jobs:
         uses: actions/setup-node@v2
         with:
           node-version: "${{ env.NODE }}"
+          cache: npm
 
       - name: Set up Ruby
         uses: ruby/setup-ruby@v1