use mirror method with apt during acceptance tests

https://mvogt.wordpress.com/2011/03/21/the-apt-mirror-method/

something like this should work :

deb mirror://mirrors.ubuntu.com/mirrors.txt bionic main restricted universe
deb mirror://mirrors.ubuntu.com/mirrors.txt bionic-updates main restricted universe
deb mirror://mirrors.ubuntu.com/mirrors.txt bionic-security main restricted universe