testpilot: prevent using tpx on Windows
testpilot: prevent using tpx on Windows

Summary:
TPX on Windows is being worked on, but not yet ready for prime time. Let's make
sure that getdeps isn't trying to use TPX on Windows for now.

Reviewed By: splhack

Differential Revision: D37474152

fbshipit-source-id: e636b0b68f6f73472f28295c280803ac44fafebf

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