misc: expose vs env vars to qmake command execution on windows

This patch prevents client build failure on systems without the
specific build toolchain path in the system or user path environment
variable.

It so happens that qmake needs the paths for the ms build toolchain,
so we pass them to the executing subprocess.

Gitlab: #581
Change-Id: Id5dcaead487d0c128c9fb04ca22d79438531b0c0