Use preinstalled Yarn on Appveyor (jestjs#5548)

* Use preinstalled Yarn on Appveyor

* Simplify config

Author: thymikee

appveyor.yml

@@ -10,17 +10,15 @@ init:
 install:
   - ps: Install-Product node $env:nodejs_version x64
   - node --version
-  - curl -fsSL -o yarn.js https://github.com/yarnpkg/yarn/releases/download/v1.3.2/yarn-1.3.2.js
-  - node ./yarn.js --version
-  - node ./yarn.js install
-  - node ./yarn.js run build
+  - yarn
 
 cache:
   - node_modules
   - .eslintcache
+  - "%LOCALAPPDATA%\\Yarn"
 
 test_script:
-  - node ./yarn.js run jest --color
+  - yarn jest --color
 
 # Don't actually build.
 build: off