Turn off nodeReuse (nr:False) for msbuild to avoid locking issues

I've been running into locking problems with msbuild.exe holding onto assemblies. It turns out that by default now msbuild.exe stays running because of this setting [nodeReuse (default:True)](http://msdn.microsoft.com/en-us/library/ms164311.aspx). When set to false the process(s) will exit after the build is complete. 

Note: Visual Studio also [does this by default](http://stackoverflow.com/questions/12174877/visual-studio-2012-rtm-has-msbuild-exe-in-memory-after-close) and you can override it by setting this environment variable. 

```
MSBUILDDISABLENODEREUSE=1
```