Comment # 11 on bug 965564 from Aleksa Sarai

(In reply to Ludwig Nussel from comment #10)
> systemd sets up /sys/fs/cgroup/pids/init.scope specifically for pid 1. So it
> could be argued that the bug is that systemd doesn't fully clean up after
> itself when switching to the real root, assuming pid 1 there is also systemd.

I'm not sure if you'll be able convince the systemd guys that this is a bug.
However, here's a simple workaround to stick in the start of the build scripts:

% echo $$ > /sys/fs/cgroups/pids/cgroup.procs

Since the root cgroup doesn't allow for pids limits, attaching to the root
cgroup should solve your problems. We could also increase the ulimits here if
appropriate.