Simplify xcode jobs CI
What does this MR do?
While working on #46 (closed) I found that the way we currently setup the Xcode image builds was a bit hard to understand. It uses templating, job triggers and child pipelines which seem like a very smart solution to reduce repetition.
I'm a big proponent of KISS and keeping internal tools the less "surprising" possible, often at the expense of DRY. Here I think repeating a few lines of .gitlab-ci.yml contents is the better compromise for mental load. It will be easier to understand when, in 6 months, we'll have a change to make and will have forgotten everything about this repo! Not using child pipelines also makes working with them in the Gitlab UI less tedious
Xcode jobs are identical to their previous templated equivalent, and you can test this easily by running them manually.