Hi Andy,
How is the variable WorkspaceRoot set? If you want it to contain the Continua CI agent build workspace path, then you need to pass the value into FinalBuilder from Continua. You can do this by adding WorkspaceRoot=$Workspace$ to the Variables list under the Variables tab of the FinalBuilder action in Continua. The same applies if you are setting it to another folder such as your TFS workspace folder. e.g. WorkspaceRoot=\\MachineName\TFSWorkspaceFolder
Why not use the TFS repository in Continua to get the source code to your workspace? Continua repositories are designed to automatically check out the code for each revision in the background and seamlessly transfer your source code to each agent when you run a build. After setting up a Continua repository you can access the repository root folder using $Source.RepoName$ and pass this to your FinalBuilder action by adding WorkspaceRoot=$Source.RepoName$ to the Variables list.