Attaching the patch here for visibility.

In short, rather than relying on PATH to find hg.exe, we now precalculate it and pass it in to the build. This completely avoids the problem where modifying PATH multiple times for different builds was causing Mercurial to fall off.

This patch also fixes two other issues with my build script - the PGOOPTS need to come before CERTOPTS, as they are parsed by different scripts, and the debug build command fell off somewhere which resulted in the debug builds not being rebuilt for 3.6.0b4 - they are still the 3.6.0b3 debug build.