Fix duplicate entries in HEADER_SEARCH_PATHS when running react_native_post_install script (#46262)

Summary:
In a react native project where USE_FRAMEWORKS is not nil, every time when running `pod install`, duplicate lines are added to `HEADER_SEARCH_PATHS` section of `project.pbxproj`:

```
" ${PODS_CONFIGURATION_BUILD_DIR}/ReactCommon/ReactCommon.framework/Headers",
" ${PODS_CONFIGURATION_BUILD_DIR}/React-Fabric/React_Fabric.framework/Headers/react/renderer/components/view/platform/cxx",
" ${PODS_CONFIGURATION_BUILD_DIR}/React-graphics/React_graphics.framework/Headers",
" ${PODS_CONFIGURATION_BUILD_DIR}/ReactCommon/ReactCommon.framework/Headers",
" ${PODS_CONFIGURATION_BUILD_DIR}/React-Fabric/React_Fabric.framework/Headers/react/renderer/components/view/platform/cxx",
" ${PODS_CONFIGURATION_BUILD_DIR}/React-graphics/React_graphics.framework/Headers",
" ${PODS_CONFIGURATION_BUILD_DIR}/ReactCommon/ReactCommon.framework/Headers",
" ${PODS_CONFIGURATION_BUILD_DIR}/React-Fabric/React_Fabric.framework/Headers/react/renderer/components/view/platform/cxx",
" ${PODS_CONFIGURATION_BUILD_DIR}/React-graphics/React_graphics.framework/Headers",
```

Note: a popular library that needs `use_frameworks` is react-native-firebase.
See https://rnfirebase.io/#altering-cocoapods-to-use-frameworks

## Analyse

- `react_native_post_install` calls `ReactNativePodsUtils.update_search_paths(installer)`
- when `ENV['USE_FRAMEWORKS'] != nil` then `update_search_paths` calls `add_search_path_if_not_included`
- `add_search_path_if_not_included` checks if `"#{new_search_path}"` is already there
- if not found it adds `" #{new_search_path}"` _with an extra space_
- next time, it can't find `"#{new_search_path}"` because of the extra space, and adds `" #{new_search_path}"` again

## Changelog:

[IOS] [FIXED] - react_native_post_install script no longer adds duplicate entries to HEADER_SEARCH_PATHS

Pull Request resolved: https://github.com/facebook/react-native/pull/46262

Test Plan:
- create a react native project
- add `use_frameworks! :linkage => :static` to `ios/Podfile` (just before `use_react_native`)
- run `pod install`
- assert no duplicate lines are added to HEADER_SEARCH_PATHS of file `project.pbxproj`

Reviewed By: cipolleschi

Differential Revision: D61982680

Pulled By: shwanton

fbshipit-source-id: 61b566893c551d0813edd6eec2f8352c041c748f