Update yarn.json

* use env path, then Git-Bash (git-for-windows) can also use yarn.

bucket/yarn.json

@@ -13,7 +13,7 @@
     },
     "url": "https://yarnpkg.com/downloads/0.23.3/yarn-0.23.3.msi",
     "hash": "23f1481674d4d735c1961a684f5d252b7b47e6f53b68e6a295ea8a93aca4957d",
-    "bin": "Yarn\\bin\\yarn.cmd",
+    "env_add_path": "Yarn\\bin",
     "checkver": {
         "url": "https://yarnpkg.com/latest-version",
         "re": "([\\d.]+)"