add SWIFT_ENABLE_EXPLICIT_MODULES to xcode 26 (#53457)

Summary:
XCode 26 introduces building explicit swift modules turned on (SWIFT_ENABLE_EXPLICIT_MODULES). This breaks building with precompiled binaries.

This commit fixes this by adding a step when not building from source where we explicitly set the `SWIFT_ENABLE_EXPLICIT_MODULES` flag to `NO`.

## Changelog:

[IOS] [FIXED] - Added setting SWIFT_ENABLE_EXPLICIT_MODULES=NO when using precompiled to support Xcode 26

Pull Request resolved: https://github.com/facebook/react-native/pull/53457

Test Plan:
```bash
npx react-native-community/cli init MyApp --version nightly --skip-install
cd MyApp
yarn
cd ios
bundle install
RCT_USE_RN_DEP=1 RCT_USE_PREBUILT_RNCORE=1 bundle exec pod install
```

Build above app with Xcode 26 and verify that it no longer fails

Reviewed By: motiz88

Differential Revision: D81025367

Pulled By: cipolleschi

fbshipit-source-id: 1db7c4d7de07d62f43b355aa784d7d9de478023c