Fix Zebra's failing coverage CI

[teor2345]

Is your feature request related to a problem? Please describe.

Zebra's coverage CI is consistently failing.

For example:
bd03a51
https://github.com/ZcashFoundation/zebra/runs/1826774021

Describe the solution you'd like

We disabled the coverage CI temporarily in #1688.

• investigate the cause of the failures
• fix the failures
• re-enable the coverage CI
• name it explicitly 'nightly'
• Check that we're not exceeding the 20 concurrent jobs limit: https://docs.github.com/en/actions/reference/usage-limits-billing-and-administration#usage-limits

Additional context

The docs CI is also failing.

[teor2345]

I've merged in the task list from #1711, and closed that ticket