buildscripts/1.2.x: dereference the 'latest-1.2.x' junction point in the Win32 build script.

Paths used in the build MUST NOT use C:\MumbleBuild\latest-1.2.x directory.

If they do, PDBs can point to the wrong files, and all our build products
will have the C:\MumbleBuild\latest-1.2.x string embeded into them, instead
of the actual build directory.