Add `alr` coverage testing to the GitHub workflows

[mosteo]

As the title says.

@Fabien-Chouteau, I was playing with this and hit a wall and the best way I can describe it is that some instrumented code was trying to call gnatcov generated code outside of project closures (in dependencies). I tried to follow the instructions in the SPARK workflow demo repo. Wondering if you had tried this.

I can provide more details at a later time.

[Fabien-Chouteau]

Last time I checked GNATcov was not able to instrument the Alire code, so that could be an issue. Otherwise it could work like in the Ada SPARK workflow.

[pjljvandelaar]

I would love the feature!