unable to reconfigure after update

[Kristoffer Ström]

I just updated to the new post-merge system and i'm running into some
strange issues.

As i run the reconfigure, it seems to switch early to a broken system,
and then attempting to bulid additional things, which fail with not
finding the guixbuild group.

Log below:

------------------------------------------------

activating system...
The following derivation will be built:
  /gnu/store/3i3fg5kn6x414aan5fhy3z0z7xgg955j-switch-to-system.scm.drv

building /gnu/store/3i3fg5kn6x414aan5fhy3z0z7xgg955j-switch-to-system.scm.drv...
making '/var/guix/profiles/system-69-link' the current system...
setting up privileged programs in '/run/privileged/bin'...
populating /etc from /gnu/store/xk3pxlmkrdgh4p1bhz40iwzbws2max82-etc...
The following derivation will be built:
  /gnu/store/r7d6jl2j9dnsjd9d1h43m7p1piwjzg7y-install-bootloader.scm.drv

guix system: error: the group `guixbuild' specified in
`build-users-group' does not exist

------------------------------------------------

any idea what has happened here?