[bug#68242] [core-updates] Compress man pages using zstd

[Ludovic Courtès]

Maxim Cournoyer <maxim.cournoyer@gmail.com> skribis:

> There were sometimes out of memory errors on the Berlin build farm, especially
> for i686 or arm machines having less memory.
>
> * guix/build/utils.scm (%xz-parallel-args): Reduce --memlimit value from 50%
> to 20%.
>
> Change-Id: If848bed92ef4c42d11a96057e59ee51a019d0573

LGTM.