release-upgrade does not work for the client behind proxy

Hi,

In an offline environment a client machine needs a proxy for external connections, even if the proxy configuration is done for apt or a system-wide (environment) or for the landscape client in client.conf, when we trigger a release upgrade via Landscape GUI, the client tries to connect archive.ubuntu.com directly, and the upgrade fails.

root@landscape-test:/var/log/landscape# tail -f release-upgrader.log
2022-11-10 10:13:12,363 INFO [MainThread] Queuing message with release upgrade failure to exchange urgently.
2022-11-10 10:13:12,364 WARNING [MainThread] Couldn't fetch file from http://archive.ubuntu.com/ubuntu/dists/focal-updates/main/dist-upgrader-all/current/focal.tar.gz (Error 7: )
2022-11-10 10:17:43,952 WARNING [MainThread] Couldn't fetch file from http://archive.ubuntu.com/ubuntu/dists/focal-updates/main/dist-upgrader-all/current/focal.tar.gz (Error 7: )
2022-11-10 10:17:43,952 WARNING [MainThread] Couldn't fetch all upgrade-tool files

We have a customer with this scenario (case 347803). I'm not sure, but maybe it could be added checking the proxy environments before starting the upgrade process.

Regards,
Ebrar.