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.

react-native/packages/react-native/scripts/react_native_pods.rb

Line 239 in 899e7cd

fabric_enabled = ReactNativePodsUtils.has_pod(installer, 'React-Fabric')

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.

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[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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