Update ios pod post_install logic for detecting if hermes is enabled (#41286)

Summary:
Follow up of #41284 (comment)

We should not rely on  checking if the `React-hermes` pod is present to determine if hermes is enabled

## Changelog:

[IOS] [CHANGED] - Update ios pod post_install logic for detecting if hermes is enabled

Pull Request resolved: #41286

Test Plan: Run `use_react_native!(hermes => false)` should not add `USE_HERMES = true;` to `project.pbxproj`

Reviewed By: blakef

Differential Revision: D50899654

Pulled By: cipolleschi

fbshipit-source-id: a5ab5b0117c61014e77b780c50bf349da92c6342

Author: gabrieldonadel

packages/react-native/scripts/react_native_pods.rb

@@ -294,7 +294,7 @@ def react_native_post_install(
   end
 
   fabric_enabled = ENV['RCT_FABRIC_ENABLED'] == '1'
-  hermes_enabled = ReactNativePodsUtils.has_pod(installer, "React-hermes")
+  hermes_enabled = ENV['USE_HERMES'] == '1'
 
   if hermes_enabled
     ReactNativePodsUtils.set_gcc_preprocessor_definition_for_React_hermes(installer)