switch to yarn in appveyor (#20566)

Summary:
switch to yarn to keep consistent. Windows android  build pass: https://ci.appveyor.com/project/gengjiawen/react-native/build/job/xyof6052yg2uhnmw.
pass all current ci.
none
 [GENERAL] [INTERNAL] [CI] - switch to yarn.
Pull Request resolved: https://github.com/facebook/react-native/pull/20566

Differential Revision: D9213415

Pulled By: hramos

fbshipit-source-id: 35183c143f831e29aa0d0c0b90baacb6a7cd7837

appveyor.yml

@@ -32,8 +32,8 @@ install:
 
     - ps: Install-Product node $env:nodejs_version
     - node --version
-    - npm --version
-    - appveyor-retry npm install
+    - yarn --version
+    - appveyor-retry yarn install
 
 build_script:
   - gradlew.bat RNTester:android:app:assembleRelease