chore(azure): node v4 compatible yarn (#5766)

Workaround for yarnpkg/yarn#6900

.azure-pipelines/steps/install-nodejs.yml

@@ -3,3 +3,8 @@ steps:
     inputs:
       versionSpec: "$(node_version)"
     displayName: "Install Node.js"
+
+  # workaround for https://github.com/yarnpkg/yarn/issues/6900
+  - script: npm install -g yarn@1.12.3
+    displayName: "Install Node v4 compatible Yarn"
+    condition: eq(variables.node_version, 4)