Improve multicore build

Only run GenerateWixInclude.exe once.
Both are built before anything that consumes the output file. There's already a GenerateWixInclude target (tools\WixBuild.Tools.targets) and I wouldn't mind taking it out of the build path -- GenerateWixInclude is called 144 times during a full build. Granted, only 106ms...But we only call this once.