Ant Task locks build log file. Unable to delete build.

Running Jenkins on a Windows 2008 server, I'm experiencing an issue where using the AntExec plugin and executing the ant unzip task on a file in the workspace works, but leaves a file handle open on the log file of the build. As a result, trying to delete these builds fails with a Java IOException see error screnshot.

Using Windows Process Explorer, I can see that the file handles are open for all the builds executed and not just the last one.

Note, I have also noticed similar issues with trying to run the same Ant task from a System Groovy script.