APT pinning broken for stretch-backports

The APT pinning for stretch-backports is ignored, because we set the origin to “Debian” instead of “Debian Backports”. I see that this was done on purpose in 8efc9e33, because our snapshots repo uses “Debian” as its origin. But in config/chroot_local-includes/lib/live/config/1500-reconfigure-APT, we change the repo to the official Debian repo, which uses “Debian Backports” as the origin for backports.

I see two solutions: Use a different archive for backports in our repo, which uses “o=Debian Backports”. Or replace the origin for the backport pins in config/chroot_local-includes/lib/live/config/1500-reconfigure-APT.

Feature Branch: segfault:bugfix/15973-fix-backports-apt-pinning

Related issues

Blocked by #15837 (closed)
Blocks #15334 (closed)

Original created by @segfault on 15973 (Redmine)

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