add yarn to android build

Author: cesar-sosa-hol

.github/workflows/main.yml

@@ -38,6 +38,7 @@ jobs:
       - name: Install npm dependencies  
         uses: actions/checkout@v2
       - run: |
+          yarn
           cd example-expo && yarn
           npx expo prebuild -p android
           cd android && ./gradlew clean build