Fix ios pod post_install logic for detecting if fabric is enabled

[gabrieldonadel]

Summary:

There is a problem in the way that we check if Fabric is enabled inside react_native_post_install.

https://github.com/facebook/react-native/blob/899e7cdb55197fc17a96a93af4f8bcc7519553c2/packages/react-native/scripts/react_native_pods.rb#L239

We're determining if fabric is enabled by checking if the React-Fabric pod is present, but since we always call setup_fabric!(:react_native_path => prefix) (#39057) inside use_react_native the React-Fabric pod is always present causing the -DRN_FABRIC_ENABLED flag to always be added to project.pbxproj even if the new arch is disabled.

Changelog:

[IOS] [FIXED] - Fix ios pod post_install logic for detecting if fabric is enabled

Test Plan:

Run use_react_native!(fabric => false) should not add the -DRN_FABRIC_ENABLED flag to project.pbxproj

[cipolleschi]

Perfect! Thank you @gabrieldonadel!

[cipolleschi]

For another change, we should probably do the same for Hermes, as line 100 sets ENV['USE_HERMES'], while line 240 decides whether we are using hermes based on the installed pods.

[facebook-github-bot]

@cipolleschi has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[gabrieldonadel]

For another change, we should probably do the same for Hermes, as line 100 sets ENV['USE_HERMES'], while line 240 decides whether we are using hermes based on the installed pods.

Here is the followup pr #41286

[github-actions]

This pull request was successfully merged by @gabrieldonadel in 0f8a83e.

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