bst doesn't use an open workspace for a junction

I have a junction with source in git, for which I opened a workspace.

• bst build doesn't use the workspace, and warns about that. It even complains when the ref is incorrect.
• bst track base/freedesktop-sdk-junction.bst warns that the updated ref won't be used (which isn't true, but is what it should be).

So there are two things that need fixing: use the workspace when available, and don't warn that it isn't used (if the warning is still present when the first part is fixed).