building build images fail by non-existing sbuild group

build images are currently failing with this error:

STEP 16/16: RUN useradd --gid 100 -G sbuild -m salsa-ci
useradd: group 'sbuild' does not exist
Error: building at STEP "RUN useradd --gid 100 -G sbuild -m salsa-ci": while running runtime: exit status 6

https://salsa.debian.org/salsa-ci-team/pipeline/-/jobs/8433416#L1972

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