use pod installed by bundler if possible

[buschco]

Summary

see https://github.com/react-native-community/cli/issues/2668 and https://github.com/react-native-community/cli/issues/2663 (should fix both).

This change ensures that local installed cocoapods (bundle exec pod) is used before system installed pod.

Test Plan

A: Bundled Version:

1. make sure no cocoapods is not installed globally: gem uninstall cocoapods
2. npx @react-native-community/cli@latest init MyApp
3. press n for cocoa pods question
4. cd MyApp
5. bundle install

Now bundle exec pod --version has an 0 exit code.

npx react-native run-ios --force-pods will not fail nor install cocoapods globally (pod will have exit code > 0)

B: Global Version (someone could argue this should never be supported anyway):

1. make sure cocoapods is installed globally: gem install cocoapods
2. npx @react-native-community/cli@latest init MyApp
3. press n for cocoa pods question
4. cd MyApp
5. remove cocoapods from Gemfile
6. bundle install

Now bundle exec pod --version has an non 0 exit code.

npx react-native run-ios --force-pods will not fail and install cocoapods globally (pod will have non 0 exit code)

Checklist

• [x] Documentation is up to date.
• [x] Follows commit message convention described in CONTRIBUTING.md.
• [x] For functional changes, my test plan has linked these CLI changes into a local react-native checkout (instructions).