Assume Buck by default when building the fixtures
Assume Buck by default when building the fixtures

Summary:
[Thrift] Assume Buck by default when building the fixtures.

This provides a new default for the build-dir argument to the build-fixtures script, matching the default build-dir that Buck uses.

Makes it ever so slightly easier to run this script.

Reviewed By: alandau

Differential Revision: D3460411

fbshipit-source-id: 165d5d48414e1402475aad7e5bf6fc6b0e0e2bf3

facebook/fbthrift,LinusU/fbthrift,LinusU/fbthrift,facebook/fbthrift,facebook/fbthrift,getyourguide/fbthrift,facebook/fbthrift,LinusU/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,LinusU/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,getyourguide/fbthrift,LinusU/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,getyourguide/fbthrift,facebook/fbthrift,SergeyMakarenko/fbthrift,facebook/fbthrift