chore(example): use prebuilt React Native for iOS on CI in FabricExample (#3955)

## Description

Speed up the build process from ~6min to ~3min (macos-xlarge-latest)

I briefly checked debugging and it seems that debug symbols aren't
available in XCode - meaning that for some more advanced debugging
sessions we'd have to locally rollback to building from source.

If this change proves to be counterproductive we can revert it later. It
will significantly speed up the builds on CI, in local development it
can be easily switched off

same in react-native-reanimated:
software-mansion/react-native-reanimated#8983

## Changes

- Enabled uising prebuilt react-native via env variables in Podfile

## Test plan

build FabricExample and try to debug something

<img width="1275" height="868" alt="Zrzut ekranu 2026-04-29 o 18 08 32"
src="https://github.com/user-attachments/assets/c5f9ce89-7888-4936-a04e-90e8dd2e98db"
/>

## Checklist

- [ ] Included code example that can be used to test this change.
- [ ] For visual changes, included screenshots / GIFs / recordings
documenting the change.
- [ ] For API changes, updated relevant public types.
- [ ] Ensured that CI passes

---------

Co-authored-by: Copilot Autofix powered by AI <[email protected]>

Author: radoslawrolka

FabricExample/ios/Podfile

@@ -11,8 +11,10 @@ require_relative '../../scripts/ios/rns_set_swift_compilation_flags'
 platform :ios, min_ios_version_supported
 prepare_react_native_project!
 
-# Disable precompiled binaries for RN core
-ENV['RCT_USE_PREBUILT_RNCORE'] = '0'
+# Use prebuilt binaries in CI, source in local development
+ci_enabled = %w[1 true].include?(ENV['CI'].to_s.downcase)
+ENV['RCT_USE_PREBUILT_RNCORE'] ||= ci_enabled ? '1' : '0'
+ENV['RCT_USE_RN_DEP'] ||= ci_enabled ? '1' : '0'
 
 linkage = ENV['USE_FRAMEWORKS']
 if linkage != nil