CI: Re-enable Souffle installation

[jubnzv]

It was disabled in #40 because of the libffi dependency mismatch between Souffle's PPA and the Ubuntu repository on Matrix OS.

[jubnzv]

Fixed in Misti by downgrading Ubuntu in Matrix OS: nowarp/misti#253
Created the following issue to get a feedback about the problem: souffle-lang/souffle#2527