Update gems:frozen script to use env var

- avoid side effect deploy mode persistant config

Ref: react-native-community/template#209

Author: leotm

package.json

@@ -14,7 +14,7 @@
     "android:prod": "yarn gradle:clean && react-native run-android --mode release",
     "gradle:clean": "cd android && ./gradlew clean",
     "gems": "bundle check || bundle install",
-    "gems:frozen": "bundle config set --local deployment true && bundle install",
+    "gems:frozen": "BUNDLE_DEPLOYMENT=true bundle install",
     "pods": "cd ios && bundle exec pod install",
     "pods:frozen": "cd ios && bundle exec pod install --deployment",
     "pods:clean": "rm -rf ios/Pods && cd ios && bundle exec pod install",