Explicitly set path to yarn

Summary:
Due to issues with Circle's Docker images ([link](https://twitter.com/circleci/status/974694807091073024)), jobs are failing with an error "yarn not found".

Test Plan
Run on Circle

Release Notes
[GENERAL][MINOR][.circleci] - Fix Circle issue
Closes https://github.com/facebook/react-native/pull/18419

Differential Revision: D7312052

Pulled By: hramos

fbshipit-source-id: 2768b9c69046a2f80518430024d3e7afbbd7de65