is there anything in Guix that can do things at install time, like postinst scripts in Debian?
A program in a package I created is trying to access /var, but has no permissions. (Well, /gnu/store/.../var)
The 'install' make target was doing 'chgrp' and 'install -g', but I had to remove these because there was no such group in the chroot jail. And it would be wrong anyway, as the gid in the jail would be different from that in the systems where the package will be installed.
So, I guess /gnu/store/.../var has to be chgrp-ed during installation, on the user's system.