ci: add workaround for Node.js legacy versions

Author: UlisesGascon

.github/workflows/ci.yml

@@ -25,6 +25,10 @@ jobs:
         with:
           node-version: ${{ matrix.node-version }}
 
+      - name: Install specific npm version for Node.js 6 and 7
+        if: matrix.node-version == 6 || matrix.node-version == 7
+        run: npm install -g npm@6
+
       - name: Install dependencies
         run: npm install