evaluate qbsSearchPaths after the item has been fully set up

This makes sure that builtin functions like qbs.getenv can be called in
qbsSearchPaths.

Task-number: QBS-463
Change-Id: I74d50f25fd7e62663a5680741b22bc27db8ea80f
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>