Summary:
Custom metro port not working without exporting variables from `.packager.env`
## Changelog
[General] [Fixed] - Fix support for custom port
Pull Request resolved: https://github.com/facebook/react-native/pull/32666
Test Plan: run `react-native run-android --port=8082` and `react-native run-ios --port=8082`
Reviewed By: yungsters
Differential Revision: D32694233
Pulled By: cortinico
fbshipit-source-id: 42e803d56b83608024b901d2a3024733ed7099b7