This resolves a minor issue where build scripts could've acquired more
job server tokens from Cargo than allowed by `-j` parameter. I've filled
a PR at https://github.com/rust-lang/cc-rs/pull/878 and we've iterated
on the design over there since.

TL;DR: some build scripts may complete a tad bit quicker, potentially
shaving off a few seconds off of debug/release builds. Full description
of the issue is available in
https://github.com/rust-lang/cc-rs/issues/858

Release Notes:

- N/A