automation: correct the path separator in LIBPATH on Windows
I haven't tried building the x86 installer, but happened to notice this when
working on the thg installer. Experimenting in PowerShell seems to show that
LIBPATH was expanded at the end, but with ':' between, it effectively corrupted
`${root}\WinSDK\Lib` and the first path in LIBPATH.
Differential Revision: https://phab.mercurial-scm.org/D6642