update npm on node@6

Author: mansona

.github/workflows/nodejs.yml

@@ -41,6 +41,8 @@ jobs:
     - uses: actions/setup-node@v4
       with:
         node-version: ${{ matrix.node }}
+    - run: npm i -g npm@6
+      if: matrix.node == 6
     - name: install dependencies
       run: npm install
     - name: test