Add CGO flags for building on Windows

In the v4.0 server branch, I noticed that the enterprise Windows build variant set the following environment variable: CGO_CFLAGS="-D_WIN32_WINNT=0x0601 -DNTDDI_VERSION=0x06010000"

We don't set that in the tools repo builds, nor it it set in the server's non-enterprise build variant. We need to understand if this is necessary and under what circumstances.