David Yack, one of our MS Regional Directors and a .NET MVP, tells me there is considerable confusion around the licensing requirements for the software installed on Team System build machines. The best resource available for these sorts of questions is the VSTS Licensing Whitepaper which goes into numerous licensing scenarios in depth. In the case of if you need a separate license of the Team System Development or Test edition on your build machine, in most cases you don't. Here's the excerpt from the whitepaper:
Those users who are authoring tests and including them as part of a Team Build type must be appropriately licensed with the relevant Team Edition product. For unit tests, Team Edition for Software Developers or Team Edition for Database Professionals or Team Edition for Software Testers or Team Suite would be required by the test author. For load and other test types, Team Edition for Software Testers or Team Suite would be required. Similarly, a user who includes code analysis in a Team Build Type must also be appropriately licensed for Team Edition for Software Developers. Users who merely execute and review Team Builds are only required to have a Team Foundation Server CAL.
The way I think about it, the people that are using the Team editions need to be properly licensed which in turn ensures the that the build machines are covered as well.
Hope this helps in some small way to reduce the confusion on this topic.