[buck2] Fix path issues that cause buck2 to think all flow unit tests pass
[buck2] Fix path issues that cause buck2 to think all flow unit tests pass

Summary: Changelog: [internal]

Reviewed By: mvitousek

Differential Revision: D38266360

fbshipit-source-id: d1f615a6474c2fc0ad141c0af1f6e0add149adee

mroch/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,facebook/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow