Bug 1429669 - Add backports sources when building Debian packages. r=dustin

Build dependencies won't be installed from backports unless they are not
satisfiable in the given Debian release. This is useful to get dh-python
on Wheezy.

--HG--
extra : rebase_source : 1f249b4ceae4fdd9ea37e9a9b9e9b62b48a1c9ed

taskcluster/taskgraph/transforms/job/debian_package.py

@@ -112,6 +112,8 @@ def docker_worker_debian_package(config, job, taskdesc):
         '/{snapshot}/ {dist} main" > /etc/apt/sources.list && '
         'echo "deb http://snapshot.debian.org/archive/debian'
         '/{snapshot}/ {dist}-updates main" >> /etc/apt/sources.list && '
+        'echo "deb http://snapshot.debian.org/archive/debian'
+        '/{snapshot}/ {dist}-backports main" >> /etc/apt/sources.list && '
         'echo "deb http://snapshot.debian.org/archive/debian-security'
         '/{snapshot}/ {dist}/updates main" >> /etc/apt/sources.list && '
         # Install the base utilities required to build debian packages.