fix: renamed postinstall script for iOS to avoid running in linux/windows

[vinitraj10]

We have postInstall script mentioned in package.json, which is by definition meant for iOS pods install, but it's not always the case since if someone running npm install on Linux/ windows machine would run into following issue mentioned in the screenshot.

The reason for that npm follows the order to run the pre and post script for every command run in the terminal like this npm word, which means the order of script call will be like this npm preword , npm word, npm postword. more about this can be found here in the npm doc.

To avoid this conflict, this PR aims to rename the pod install script from post-install so that it won't run in Linux machine unncessarily