Default autogroup niceness of Guix build daemon

["J. R. Haigh (re. Guix)" <JRHaigh+ML.GNU.Guix@Runbox.com>]

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.
P.s.: Although I've been on this list for just under a year, this is my first email to the list – hi everyone! Maybe see you again at FOSDEM 2020.
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