ci: use npm 6.x on Node.js 10 CI jobs

Latest npm is now 7.x and 7.x appears to cause CI failures when setting up the E2E test framework on Node.js 10.

(cherry picked from commit dfc6d89)

Author: clydin

.circleci/config.yml

@@ -207,8 +207,8 @@ jobs:
           name: Initialize Environment
           command: |
             ./.circleci/env.sh
-            # Ensure latest npm version to support local package repository
-            PATH=~/.npm-global/bin:$PATH npm install --global npm
+            # Ensure latest v6 npm version to support local package repository
+            PATH=~/.npm-global/bin:$PATH npm install --global npm@6
       - run: PATH=~/.npm-global/bin:$PATH node ./tests/legacy-cli/run_e2e --nb-shards=${CIRCLE_NODE_TOTAL} --shard=${CIRCLE_NODE_INDEX}
 
   test-browsers: