Convert CI to use matrix builds

Our CI setup uses an implicit matrix to build, assemble and test our code against different Go, Git and Ruby versions. In order to not repeat ourselves, we thus used YAML anchors to merge in a common definition, which already avoids quite some maintenance burden. But the end result is still a tad hard to read as it's easy to miss the differences between jobs.

Let's migrate to parallel matrix jobs instead. This new feature which has been introducen in GitLab v13.3 allow us to do exactly what we want: given a matrix of variables, run the job with each combination. This allows us to get rid of some of the YAML anchors and explicitly states what we're iterating over, improving maintainability.