change workflow to use npm 6

.github/workflows/deploy-prod.yml

@@ -13,11 +13,13 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
-      - name: Set up Node.js version 14
+      - name: Set up Node.js
         uses: actions/setup-node@v2
         with:
           node-version: '14'
           cache: 'npm'
+      - name: Install specific npm version
+        run: npm install -g npm@6.14.18
       - run: npm install
       - run: CI=false npm run build
       - uses: FirebaseExtended/action-hosting-deploy@v0