CI: use npm ci on Node.js >=8.

Author: XhmikosR

.github/workflows/test.yml

@@ -26,8 +26,14 @@ jobs:
       - run: node --version
       - run: npm --version
 
-      - name: Install npm dependencies
-        run: npm install # switch to `npm ci` when Node.js 6 support is dropped
+       # remove this block when Node.js 6.x is dropped
+      - name: Install npm dependencies (npm i)
+        run: npm install
+        if: matrix.node < 8
+
+      - name: Install npm dependencies (npm ci)
+        run: npm ci
+        if: matrix.node >= 8
 
       - name: Run lint
         run: npm run lint