Implicit jobserver token is yielded by rayon #67385
Description
Rayon is currently yielding the implicit token "owned" by the process which means we can end up in a situation where e.g. a -j1 build has more than 1 rustc running (even though only one of them is actively running, but they're all consuming memory and other resources, so this is a bug).
The current intended fix is to stop yielding the implicit token by keeping track of the amount of tokens we've acquired/yielded and just skipping the yield (and eventual reacquire) for that implicit token.
cc #64750