getdeps: use c:/open/scratch if available on windows
getdeps: use c:/open/scratch if available on windows

Summary:
Ideally we'd be using mkscratch, but this still isn't shipped
to our Windows systems.

Pick a path that is more friendly to our corp windows environment by default.

Reviewed By: pkaush

Differential Revision: D20342277

fbshipit-source-id: c85bccee6701adc03b26c92ba217b18bd684257a

facebook/wangle,facebook/wangle,facebook/wangle