How do I change the build directory that MSBuild uses under Team Foundation Build?

<!--  BUILD DIRECTORY
 This property is included only for backwards compatibility. The build directory used for a build
 definition is now stored in the database, as the BuildDirectory property of the definition's
 DefaultBuildAgent. For compatibility with V1 clients, keep this property in sync with the value
 in the database.
-->
<BuildDirectoryPath>UNKNOWN</BuildDirectoryPath>