attempt at making CI faster

Move rust-nightly to stage test so it runs in parallel with coverage, which are the two longest jobs, and currently run in sequence.
Don't document dependencies, should make the step about 50% faster.
Use about 12M of cache to not recompile grcov each time.
Don't compile grcov with coverage, we don't need it, it's probably slower both to compile and execute (and larger to cache).

I'm tempted to use more cache for build artifacts, but I know space come at a premium lately