clean dir in ccnet prebuild task

<!--  

    Pre build tasks 

    User powershell to CLEAN the directory, it doesn't remove the parent as this will be in use by cc.net 

-->

<prebuild> 

  <exec> 

    <executable>powershell.exe</executable> 

    <buildArgs><![CDATA[-command "& {if( [System.IO.Directory]::Exists($pwd) ){dir $pwd | ri -recurse -force}}"]]></buildArgs> 

  </exec> 

</prebuild>