Fix long iOS build times

[nickmeinhold]

The long build times are apparently due to cloud_firestore:
[cloud_firestore] Xcode build extremely slow · Issue #349 · FirebaseExtended/flutterfire

As suggested, I changed to using incremental installation but then was getting build errors as described here:
Could not access pods_project on post_install hooks when incremental_installation is enabled · Issue #8604 · CocoaPods/CocoaPods

When incremental_installation is enabled, CocoaPods doesn't generate Pods.xcodeproj when there are nothing changes between cached project.
In this case PodsProjectGeneratorResult#project become nil.

So installer#pods_project also become nil.

So my workaround was just to check that pods_project is not null, ie:

post_install do |installer|
  if installer.pods_project
    installer.pods_project.targets.each do |target|
      target.build_configurations.each do |config|
        config.build_settings['ENABLE_BITCODE'] = 'NO'
      end
    end
  end
end

This seems to work but I don't know enough to be confident this is a good approach. We should be on the lookout for problems.