Add Type: non-native to skeleton 'builder-only'.

The package type declaration is closely tied to the templates being
served. This commit means that all tests relying on the 'builder-only'
skeleton will automatically declare a non-native type.

At the time of writing, that skeleton was only used by two tests for
historical debhelper templates, generic-dh-make-2005 and
generic-dh-make-2008.  Those two tests no longer need to provide their
own declarations.

Signed-off-by: Chris Lamb <lamby@debian.org>