Allow schroot test to run behind a proxy

Hi,

The Ubuntu CI infrastructure runs behind a proxy, and our tests have been broken for a while now. This MR is part of an effort to fix that.

That code has already proven to work on our infra, and the CI is also passing on Salsa, which means I probably didn't break anything.

In the discussions we had around that internally, there were suggestions to use mmdebstrap, but they wouldn't really change the general approach of using a wrapper to pass to sbuild-createchoot, as it's the main problem that we need to work around here.

What do you think of it? Is the approach at least acceptable for you? Do you have any other suggestion?

cc @paride