Add workaround for when concurrent builds are disabled to use single working directory

Changes to enable concurrent builds means that the builds directory exists for each agent.
This isn't required if the concurrent builds are turned off, so we should use the base directory.

Workaround

You can revert Bamboo and use the old build-dir directory structure, by applying the patch attached to this ticket. To apply the patch:

1. Shutdown Bamboo,
2. Create the following directory heirarchy:

   <Bamboo-Install>/webapp/WEB-INF/classes/com/atlassian/bamboo/build/fileserver
   

3. Download the DefaultBuildDirectoryManager_BAM-5918.class file attached to this ticket, and copy it to the folder from step (2).
4. Restart Bamboo.