Code review comment for ~mwhudson/germinate:apt-config-archive

So as mentioned out of band, the motivation for this is this ubuntu-cdimage change: https://code.launchpad.net/~mwhudson/ubuntu-cdimage/+git/ubuntu-cdimage-1/+merge/457014.

Passing just the .sources file would be OK, I guess, but would mean redundantly downloading the lists and require a separate configuration of a proxy and so on. Nothing very interesting but not ideal.

I didn't realise germinate had a policy of not relying on apt. I'm not sure having germinate know the layout of apt repositories independently of apt is worth this but well. At least this change should be a noop in the case that the new argument is not used.