ci: use latest npm 6.x for E2E tests (#11898)

Author: clydin

.circleci/config.yml

@@ -22,7 +22,7 @@ jobs:
           <<: *_root_package_lock_key
           paths:
             - "node_modules"
-  
+
   lint:
     <<: *defaults
     steps:
@@ -63,7 +63,7 @@ jobs:
     steps:
       - checkout: *post_checkout
       - restore_cache: *_root_package_lock_key
-      - run: npm install --global npm@6.1
+      - run: npm install --global npm@6
       - run: xvfb-run -a npm run test-cli-e2e -- --nb-shards=${CIRCLE_NODE_TOTAL} --shard=${CIRCLE_NODE_INDEX}
 
   build: