OpenSK moved its CTAP code into a library, and all fuzzing has moved
with it. As an advantage, we are now independent of other dependencies
that restricted the compiler versions we could use. Therefore, we are
now fuzzing on the latest nightly, hopefully fixing some issues that
came with the compiler restriction before.
The corresponding PR on our GitHub is
https://github.com/google/OpenSK/pull/602
* opensk: do not specify nightly
so that coverage works because we use the cargo wrapper
which looks for fuzz build argument at the beginning
* opensk: use RUSTUP_TOOLCHAIN to avoid override
kaczmarczyck