Auto merge of #140145 - Zoxc:job-server-proxy, r=SparrowLii

Add a jobserver proxy to ensure at least one token is always held

This adds a jobserver proxy to ensure at least one token is always held by `rustc`. Currently with `-Z threads` `rustc` can temporarily give up all its tokens, causing `cargo` to spawn additional `rustc` instances beyond the job limit.

The current behavior causes an issue with `cargo fix` which has a global lock preventing concurrent `rustc` instances, but it also holds a jobserver token, causing a deadlock when `rustc` gives up its token. That is fixed by this PR.

Fixes rust-lang/rust#67385.
Fixes rust-lang/rust#133873.
Fixes rust-lang/rust#140093.

Author: bors