getdeps: use half as many jobs by default on windows
getdeps: use half as many jobs by default on windows

Summary:
This takes some pressure off both cpu and memory
on a laptop.

Reviewed By: pkaush

Differential Revision: D20562474

fbshipit-source-id: a058c71c47f25c3a2b3c1e34a0d0caf83e642021

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