Checkout tarballs directly in the build dir

When populating the build dir with a tarball gbp will create a symlink (if both are different). While this works fine for the build it does mean things get confused when just the build dir is stored as an artifact.

To fix simply make the tarball and build dirs the same as there is no real benefit in keeping them seperate.

Signed-off-by: Sjoerd Simons sjoerd.simons@collabora.co.uk