[ios] Resolve `use_react_native!(path)` param in `reacft_native_pods.rb` from `Pathname.pwd`

[kitten]

Summary:

[!NOTE] This is a bit of a speculative fix that's based on trying to get a bare project with react-native-macos to link directly against it in an isolated installation, which makes passing path set to require.resolve('react-native-macos/package.json') more convenient than constructing a relative path first.

The path argument in react_native_pods.rb is not resolved but instead later joined using File.join(relative_path_from_current, path). This doesn't actually resolve the path, meaning, if path is absolute it's appended to this first relative path.

Instead, we should use Pathname#join and join the path to the pathname, guaranteeing it to be absolute, then construct relative paths (for the prefix) using relative_path_from against the installation root.

If we don't do this, the absolute path gets treated as a relative path (e.g. ./Users), which leads to errors such as:

[!] Invalid Podfile file: Couldn't find the React Native package.json file at ./Users/phil/git/expo/expo-pnpm/packages/expo/package.json.

Since absolute paths are unambiguous the change should be safe.

Changelog:

[IOS] Allow absolute react-native paths to be passed to use_react_native! in project podfiles

Test Plan:

This can be replicated by replacing path in the use_react_native call with an absolute resolution and running pod install after, for example:

    use_react_native!(
      :path => File.dirname(`node --print "require.resolve('react-native/package.json')"`),
      :hermes_enabled => podfile_properties['expo.jsEngine'] == nil || podfile_properties['expo.jsEngine'] == 'hermes',
      # An absolute path to your application root.
      :app_path => "#{Pod::Config.instance.installation_root}/..",
    )

Usually, the path argument gets resolved from the react-native-config via config[:reactNativePath]. However, this makes the implicit assumption that use_native_modules! has already converted this to a relative path.