Fetch live-boot and live-config from our own APT repository.

This branch is about upgrading Tails to live-boot 3.0~b7-1 and live-config
3.0.12-1, and may not be compatible with any version pushed to sid after the
next Tails freeze.

Let's keep the >>stable pinning for unstable though, it's a sensible fallback
for branches of ours that don't ship custom live-* packages.