Windows Nightly build error: pip install of SimpleITK build failing

[Sam_Horvath (Sam Horvath (Kitware))]

This has been happening intermittently for a while, I thought I had sorted it out earlier when I fixed the nightly build not being cleaned correctly.

Slicer build error on Windows dashboard

Coming back to this, it looks like a number of issues were occurring on the factory machine, most notably that the build directory was not being cleaned before the nightly build, which can cause any number of issues when changing major dependencies.