Use new mirror on riscv64 workers again

It seems that mirrors.ustc.edu.cn is not always good also, like: https://ci.debian.net/data/autopkgtest/unstable/riscv64/r/ruby-jekyll-feed/51331932/log.gz

Now we may switch to mirrors.tuna.tsinghua.edu.cn also meanwhile trying on apt proxy to improve the service.

Signed-off-by: Bo YU tsu.yubo@gmail.com