Fix broken coverage badge

[boolean5]

The coverage badge and the link to the coverage report is currently broken in the README of the main branch, because the last CircleCI build happened more than a month ago. We store the coverage badge as an artifact on CircleCI but CircleCI deletes artifacts after 30 days.

One way to fix this would be to schedule a CircleCI build to run every 15 days (this is an example interval, feel free to suggest another). We can configure this in the CircleCI config file. I think having a scheduled build is a good idea because it might also highlight other problems, such as intermittent failures in tests.

Alternatively, we can just manually trigger a build every time this happens or store the coverage badge somewhere else.

@say-yawn, what do you think?