wip-ungrafting builds stuck

[Ludovic Courtès]

Hi!

The ‘wip-ungrafting’ branch that Leo set up has been building for ~36h.
It was at 26% 24h hours ago and it’s now stuck at 33%, even though all
but two workers are idle.

<https://ci.guix.gnu.org/eval/22683/dashboard> shows that many x86_64
builds are missing.

Any idea what could be wrong, Mathieu?  What would you suggest to do
when investigating such issues?

Thanks,
Ludo’.