Enable coverage on all builds?

[astrofrog]

For the core astropy package, including coverage testing has a significant performance impact, but for other packages this won't necessarily be the case. We could consider adding --coverage to all builds by default instead of having a dedicated coverage build.

[pllim]

I definitely set it up by default for a few packages, so this is feasible in some cases. But maybe add a comment to discuss when you don't want to do that?

[pllim]

I think this idea was rejected in review of #438