Fix the Wheezy build in a nicer way

Fix the build for real (workaround’d in 4218f2e4): at lb source time, when lb chroot_sources install wants to run apt-get upgrade, it fails the build with The value 'wheezy' is invalid for APT::Default-Release as such a release is not available in the sources (due to the APT proxy not being configured at this time, the last apt-get update failed and the package indices are empty). If one simply removes APT::Default-Release, then the pinning is broken when indices are not available. Actually, if lb source just exited early unless LB_SOURCE is enabled, we would not have this problem.

Feature Branch: feature/8194-APT-socks

Related issues

Blocked by #8194 (closed)

Original created by @tails on 5307 (Redmine)

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