Fix: OSS builds can accidentally using coreutils cp (#44097)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/44097

There are two places where we use a feature specific to the system version of 'cp', the:

  -X    Do not copy Extended Attributes (EAs) or resource forks.

This feature isn't available in GNU's cp, which is commonly installed on macOS using:

  brew install coreutils && brew link coreutils

We can avoid the problem alltogether by being specific about the path of the system cp.

Changelog: [General][Fixed] don't break script phase and codegen when coreutils installed on macOS

Reviewed By: cipolleschi

Differential Revision: D56143216

fbshipit-source-id: f1c1ef9ea2f01614d6d89c4e9eedf43113deb80c