chore(build): upgrade Yarn (carbon-design-system#2154)

And a change to ensure a specific version of Yarn runs.

Author: asudoh

.circleci/config.yml

@@ -9,14 +9,13 @@ jobs:
       - run:
           name: Install yarn
           command: |
-            curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.12.3
-            export PATH=$HOME/.yarn/bin:$PATH
+            curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.15.2
       - run:
           name: Install dependencies from offline mirror
-          command: yarn install --offline
+          command: PATH=$HOME/.yarn/bin:$PATH yarn install --offline
       - run:
           name: Run Continuous Integration checks
-          command: yarn ci-check
+          command: PATH=$HOME/.yarn/bin:$PATH yarn ci-check
       # - deploy:
           # name: deploy to IBM Cloud
           # command: |