Code review comment for ~jslarraz/ubuntu-security-tools:add-apt-source-to-autopkgtest

MR updated according to your commentaries.

I've been experimenting a very weird behavior. The MR worked perfectly when configuring build_tools_sbuildmirror with any mirror other than main archive. However, when using the main archive, apt update didn't fetch metadata properly (I could observe a hit instead of a get in the debug info), what resulted in apt crashing when tried to download a package version that no longer exist.
(In my case binutils-common2.42-2ubuntu1, which has been replaced by version 2.42-3ubuntu1).

Forcing apt update to download all the information by clearing apt's index (i.e. rm /var/lib/apt/lists/*_*) fixed the issue, but it looks even more hacky. I wonder if you have any clue of what may be happening here.