Fix RNTester prebuild mode in iOS CI

[zeyap]

Summary

• explicitly disable RN core and dependency prebuilts for RNTester source-mode jobs
• update the local ReactNativeDependencies and React-Core-prebuilt podspecs alongside Hermes for prebuilt jobs
• preserve the intended static/prebuilt and dynamic/source matrix behavior

The stable-branch Test All workflow exposed this after the 0.87.0-rc.1 version bump: CocoaPods rejected the locally resolved prebuilt podspecs because the action only updated Hermes. Dynamic jobs also selected the published RC prebuilts despite being configured for source mode.

Stable failure: https://github.com/react/react-native/actions/runs/29740840716

Test Plan

• ./node_modules/.bin/prettier --check .github/actions/test-ios-rntester/action.yml
• verified on 0.87-stable that RCT_USE_RN_DEP=0 RCT_USE_PREBUILT_RNCORE=0 resolves both RN core and dependencies from source
• full CocoaPods probe on main was not run locally because the main bundle requires concurrent-ruby >= 1.3.7, which is not installed in the local Ruby environment

[github-actions]

Caution

Missing Changelog

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[meta-codesync]

@zeyap has imported this pull request. If you are a Meta employee, you can view this in D112810648.