Use built version of mtail

For this we will need to clone the repo (maybe use a mirror for it) and set up gitlab-ci so we can keep a binary as an artifact to use it wherever we need, without installing the go compiler.

The only thing that I hate about mtail is that there is no tagging whatsoever, so we will need to work based on a serial number for versions, or just use the commit id.

cc/ @bjk-gitlab