Suggestion: Do not change system.sources URIs and Mirrors

[hedw1gP]

tl; dr Not every Pop!_OS user is in the US.

release upgrade aborted: fetch of package failed: package fetching failed

In src/release/repos.rs system_sources(), the URL is hardcoded to always be http://us.archive.ubuntu.com/ubuntu/, which is not always a good option for people like me, who aren't in the US; actually, I am at a totally opposite site: China!

In my case, I use mirrors.tuna.tsinghua.edu.cn as my mirror, and I never got the failed message during the upgrade.

The possible way I can think:

• Read system.sources URIs;
• Let user spectify a mirror as an argument, dialog or settings.

[ningfei]

same for European user...