ci: use pod install --project-directory when testing template (#37996)

Summary:
Exercise `pod install --project-directory=ios` when building the generated iOS project to make sure we don't regress in the future.

See also https://github.com/facebook/react-native/issues/37992, https://github.com/facebook/react-native/issues/35754, https://github.com/facebook/react-native/issues/34215, https://github.com/facebook/react-native/issues/33909…

## Changelog:

[INTERNAL] [ADDED] - Exercise `pod install --project-directory=ios` when testing the iOS template

Pull Request resolved: https://github.com/facebook/react-native/pull/37996

Test Plan: CI should pass.

Reviewed By: dmytrorykun

Differential Revision: D46972815

Pulled By: cipolleschi

fbshipit-source-id: 69617b09ac599eba3dde3ddddcf08db95bfc4da3

.circleci/config.yml

@@ -904,8 +904,9 @@ jobs:
               export USE_FRAMEWORKS=dynamic
             fi
 
+            cd ..
             bundle install
-            bundle exec pod install
+            bundle exec pod install --project-directory=ios
       - run:
           name: Build template project
           command: |