Introduce isOSS (#52222)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/52222

Changelog: [Internal]
Introduce environment option to force usage of OSS fantom test runner.
If env is not set - check for BUCK file in tester which is checked in for FB but not for OSS.

Reviewed By: rubennorte

Differential Revision: D77160761

fbshipit-source-id: 1701ff140ff2be1bbeacfb4305e9f89089cacb42