Run packager from correct folder (cherry-pick of 40f816d)

[amgleitman]

Please select one of the following

• I am removing an existing difference between facebook/react-native and microsoft/react-native-macos 👍
• I am cherry-picking a change from Facebook's react-native into microsoft/react-native-macos 👍
• I am making a fix / change for the macOS implementation of react-native
• I am making a change required for Microsoft usage of react-native

Summary

This is an alternative to #1910 where we cherry-pick a commit from upstream that fixes the problem in a slightly different way.

This ensures that PROJECT_ROOT has the correct value so the JS bundler knows how to resolve things properly.

Changelog

[INTERNAL] [FIXED] - Specify PROJECT_ROOT in rn-tester yarn start script

Test Plan

Launched RNTester using the bundler from yarn start, and it works.