Use matrix for job configuration

[hoffbrinkle]

This allows us to have all the build-and-test configurations specified
in a single location (and will make it easier to matrix other
configurations).

[chiphogg]

I think we used to use a matrix, but switched to this approach. I think the reason was so that we could have separate badges? Indeed, if you click on , you see that it links to a workflow.

[hoffbrinkle]

Well that is a shame. The matrix stuff also makes nice when running the workflows locally (e.g. --matrix=config:gcc1{2,5}). There's some third party stuff out there, but we'd have to stand up a service or something. There is this discussion but not a lot of traction.

The reusable workflow thing is gives us the badges, but does result in a fair bit of boilerplate being copied around. I suppose it's the best option for now though.