Copy the block for configuring the build user and disabling man pages
from debian-bookworm-build. This fixes failures on
ttcn3-ggsn-test-kernel-latest* (non-latest worked fine since these use
containers based on debian-bookworm-build).
Fixes: 5a48bfbe ("scripts/kernel-test: build initrd+kernel as user")
Change-Id: Idee29d5b9796e8905bdf822d076f25612cbc77a0