Extension build error appears to be caused by the dashboard
SliceTracker extension is not available in the today's nightly.
Looking at the error, it does not make sense to me, since the file
cmake fails to copy is present in the repo:
http://slicer.cdash.org/viewBuildError.php?buildid=971862. The same
extension with the same hash did not have any build errors yesterday,
so it looks like the extension build error is caused by the dashboard