chore: use npm install -g yarn in buildspec (#406)

Author: trivikr

buildspec.yml

@@ -5,11 +5,11 @@ phases:
     runtime-versions:
       nodejs: 10
     commands:
-      - curl -o- -L https://yarnpkg.com/install.sh | bash -s -- --version 1.17.3
-      - export PATH="$HOME/.yarn/bin:$PATH"
-      - yarn
+      - npm install -g yarn
   build:
     commands:
+      - echo Building...
+      - yarn
       - echo Executing unit tests
       - yarn test
   post_build: