bsdtar chooses the base system tar if it is libarchive tar (NetBSD
9/current, typically), and the package otherwise, so there is no need
to conditionalize setting bsdtar on NetBSD.
(As discussed on tech-pkg@.)
Only force bsdtar if NetBSD <=8.
(This is still being discussed on tech-pkg; apparently the previous
fix was not aware of the (rapidly moving around) discussion.)