Run coverage on the latest nightly

[teor2345]

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

In #1953, we pinned our coverage build to nightly-2021-03-24. But we should really track the latest nightly. (Or beta once the features we need start to stabilise.)

Describe the solution you'd like

Revert #1953.

Describe alternatives you've considered

We could pin coverage to known good nightly versions every month, but that's a lot of work.

We could use an unsupported workaround to get the features we need on stable or beta.

[teor2345]

nightly-2021-04-14 has fixes for a number of issues on nightly.

We shouldn't be affected by rust-lang/rust#83538, because we've turned off incremental compilation.

[teor2345]

Obsoleted by #3367.