apt::Architecture seems to be unset in debbuild

[tfoote]

I noticed apt.conf looked like this:

Dir::Etc /var/cache/pbuilder-precise-amd64/apt-conf-6/etc/apt;
Dir::State /var/cache/pbuilder-precise-amd64/apt-conf-6/var/lib/apt;
Dir::Cache /var/cache/pbuilder-precise-amd64/apt-conf-6/var/cache;
Dir::State::status /var/cache/pbuilder-precise-amd64/apt-conf-6/var/lib/dpkg/status;
Apt::Architecture ;

http://jenkins.ros.org/job/ros-groovy-rqt-service-caller_binarydeb_precise_amd64/ws/apt.conf