Summary:
The migration to tpx broke Watchman's tests because test_bser relies on YARN_YARN_OFFLINE_MIRROR being set in the environment and tpx doesn't forward environment variables into the test.

Explicitly forward them ourselves.

Reviewed By: fanzeyi

Differential Revision: D27897172

fbshipit-source-id: 16c8017a89979802bd9d443825ed4e22cb6ff6c9