buildserver: fix apt Acquire::Retries syntax
Acquire is its own group, not a subgroup of APT: https://manpages.debian.org/buster/apt/apt.conf.5.en.html#THE_ACQUIRE_GROUP
Acquire is its own group, not a subgroup of APT: https://manpages.debian.org/buster/apt/apt.conf.5.en.html#THE_ACQUIRE_GROUP