bug#35139: Rust builds systematically time out

[Mathieu Othacehe]

Hello,

>>>  1. Rust takes too long to build.  What can we do about it?  Enable
>>>     parallel builds?

I've noticed that Rust packages are also built with "-j1". Evaluations
such as: https://ci.guix.gnu.org/eval/19873 are causing rebuilds of many
Rust packages, hence monopolizing the build farm for hours.

Would it be possible to enable parallel building for Rust packages as
suggested by Ludo in this thread?

Thanks,

Mathieu