Firstly, use the AfterBuild msbuild target rather than PostBuild event. This will give msbuild more information about what you're trying to do and done correctly should mean a faster incremental compile.
Environment variables can be used in AfterBuild events: http://msdn.microsoft.com/en-us/library/ms171459.aspx
Ideally once you've run msbuild once, when you run it a second time it should do no work skipping compiles and not bothering to copy things around as the files are already there.