Only build ios in check nightlies (#47506)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/47506

The Check nightlies job prepare a new nightly app with additional libraries and on iOS it builds with `yarn ios`.

The command tries to launch the app on the simulator which sometimes fails and this can make the jobs keep running for hours.

This change make sure that we only build the app

## Changelog:
[Internal] - Only build iOS in CI for Check Nightlies

Reviewed By: blakef

Differential Revision: D65656812

fbshipit-source-id: 14db3738f33f8024c9e99fe206b170209154bac7