Can we use stable rust in coverage?

[sinitcin]

When we use substrate, it required the nightly version. Now, it's not necessary.

The current version in coverage: nightly-2022-03-22, that haven't some features, for example bool_to_option feature that provide then_some function. The nightly-2022-03-22 much outdated than the current stable.

In tokens PR we are going to change version to 16-08-2022, but we have to inspect how's our coverage ready to be changed to stable version.