setup-dchroot: Request unmerged /usr

setup-dchroot: Request unmerged /usr

Merged /usr is known to cause multiple packages to be misbuilt. As long
as we support unmerged /usr for user systems, we should mitigate
this class of bugs by using unmerged-/usr chroots on official buildds,
resulting in binary packages that work equally well on merged- or
unmerged-/usr user systems.

See:

https://bugs.debian.org/913229
https://udd.debian.org/cgi-bin/bts-usertags.cgi?user=md@linux.it&tag=usrmerge
thread at https://lists.debian.org/debian-devel/2018/11/msg00299.html

Signed-off-by: Simon McVittie <smcv@debian.org>
Signed-off-by: Julien Cristau <jcristau@debian.org>