Use mirror repo instead of upstream repo

[kmaehashi]

Currently we use upstream repo but we should use downstream mirror for stability.

11:30:48 Get:1 http://security.ubuntu.com/ubuntu bionic-security InRelease [88.7 kB]
11:30:48 Hit:2 http://archive.ubuntu.com/ubuntu bionic InRelease
11:30:48 Get:3 http://archive.ubuntu.com/ubuntu bionic-updates InRelease [88.7 kB]
11:30:48 Get:4 http://archive.ubuntu.com/ubuntu bionic-backports InRelease [74.6 kB]
11:30:48 Reading package lists...
11:30:48 E: Release file for http://archive.ubuntu.com/ubuntu/dists/bionic-backports/InRelease is not valid yet (invalid for another 18s). Updates for this repository will not be applied.

cc/ @emcastillo