Default autogroup niceness of Guix build daemon

[J. R. Haigh (re. Guix)]

Hi all,
        I've been using Guix on Debian 9 Stretch for a few months, now. Besides 
various build failures, one of the worst issues is hogging of the CPU, making 
the rest of my system almost unusable for several hours (or sometimes even 
days), depending on the builds. If builds don't finish over night then I have 
had to SIGSTOP the builds in order to use my laptop, and SIGCONT them when idle 
again.
        Today, however, I've found a partial solution, after realising why 
setting process niceness to 19 does not fix the problem on my system; instead 
of the process niceness, changing the /autogroup/ niceness to 19 by issuïng the 
following command:

$ (for P in $(ps --group="guixbuild" --format="pid="); do 
F="/proc/$P/autogroup"; echo "$F"; cat "$F" && sudo tee "$F" <<<19; done)
/proc/30487/autogroup
/autogroup-10675 nice 0
19
/proc/30537/autogroup
/autogroup-10679 nice 0
19

…completely removes the lag caused by any processes in the ‘guixbuild’ 
usergroup, and allows my system to be usable for interactive work 
simultaneously with the build!!! This is great, but it only lasts as long as 
the build, and subsequent builds default to autogroup niceness 0, which is 
really not very nice!! I'd rather not run the above command in a loop, hence 
the question…
        Is there a way to declaratively set the default autogroup niceness of 
Guix's build daemon?

Regards,
James R. Haigh.
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