use merged /usr

from a mail:

AFAIK, chroots for building are already supposed to use
usr-merge from bookworm onwards.

I have this in one of my scripts that takes care
of creating chroots:

merged_option() {
  if [ "$1" = "buster" ] || [ "$1" = "bullseye" ]; then
    echo "--no-merged-usr"
  else
    echo "--merged-usr"
  fi
}

This should go into https://salsa.debian.org/lucas/collab-qa-tools/-/blob/master/bin/cqa-setup-rebuildnode