Build iOS app in New Architecture when running E2E tests. (#38813)

Summary:
Right now we're building by default Android app in New Architecture, so to align this behaviour between platforms - I added `RCT_NEW_ARCH_ENABLED=1` when installing Pods when running E2E tests, to also build iOS app in New Arch.

## Changelog:

[IOS] [CHANGED] - Build iOS app in New Architecture when running E2E tests.

Pull Request resolved: https://github.com/facebook/react-native/pull/38813

Test Plan: App in `test_e2e_ios` job should build in New Architecture

Reviewed By: dmytrorykun

Differential Revision: D48112814

Pulled By: cipolleschi

fbshipit-source-id: f709fe23f7c3ff65f3e39acb304581c4ec0fdea8

.circleci/config.yml

@@ -878,7 +878,7 @@ jobs:
             cd packages/rn-tester
             bundle check || bundle install
             bundle exec pod setup
-            bundle exec pod install --verbose
+            RCT_NEW_ARCH_ENABLED=1 bundle exec pod install --verbose
       - run:
           name: Boot iOS Simulator
           command: source scripts/.tests.env && xcrun simctl boot "$IOS_DEVICE" || true