Visual-studio – How to Set the Working Directory in NAnt

I am just getting started using NAnt. I was working from a tutorial, and just trying to set a target to clean my solution on build. My Visual Studio Solution structure is as follows:

• Solution Folder
  • Project Folder
  • Project Folder
  • Tools Folder
    • NAnt Folder

The NAnt .exe file resides in the Tools/NAnt folder. My .build file is also in there. Here is my .build file:

<?xml version="1.0" encoding="utf-8" ?>
<project name="NAntTest" default="build" xmlns="http://nant.sf.net/release/0.86-beta1/nant.xsd">
  <property name="solution.file.name" value="NAntTest.sln" />
  <property name="project.config" value="debug" />

  <target name="build" depends="clean.source" />

  <target name="clean.source">
    <exec program="${framework::get-framework-directory(framework::get-target-framework())}\msbuild.exe"
          commandline="${solution.file.name} /t:Clean /p:Configuration=${project.config} /v:q" 
          workingdir="."/>
  </target>

</project>

This is how the example I am following was formatted. If I try to run this build, I get an error stating that the project file does not exist. In the clean.source target, if I replace the workingdir attribute with a hard coded path to my base solution folder, the script compiles and runs correctly. Obviously, this is not ideal for portability if I need to move the project anywhere.

How do I get NAnt to see the base working directory?