fixed issue with xcconfig being overwritten (#54838)

Summary:
To be able to change the target xcconfig from within `add_rn_core`, we need to make sure it is not overwritten by the line that sets the config.

This commit fixes this by moving it after the line:

`spec.pod_target_xcconfig = current_config`

## Changelog:

[IOS] [FIXED] - Moved setting xcconfig to after `add_rn_core` pod function is called.

Pull Request resolved: #54838

Test Plan: Run RNTester with precompiled binaries.

Reviewed By: huntie

Differential Revision: D88849730

Pulled By: cipolleschi

fbshipit-source-id: 632e23f5a464ea779035ca3e090c0a3c9475a260

Author: chrfalch

packages/react-native/scripts/cocoapods/new_architecture.rb

@@ -135,9 +135,13 @@ def self.install_modules_dependencies(spec, new_arch_enabled, folly_version = He
 
         depend_on_js_engine(spec)
         add_rn_third_party_dependencies(spec)
-        add_rncore_dependency(spec)
 
         spec.pod_target_xcconfig = current_config
+
+        # add_rncore_dependency must be called after setting pod_target_xcconfig
+        # because it reads and modifies the xcconfig. If called before, its changes
+        # would be overwritten by the assignment above.
+        add_rncore_dependency(spec)
     end
 
     def self.extract_react_native_version(react_native_path, file_manager: File, json_parser: JSON)