gitlab-ci: fix apt conf syntax for Acquire::Retries

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

.gitlab-ci.yml

@@ -50,9 +50,9 @@ variables:
       mkdir -p apt-cache
       export APT_CACHE_DIR="$(pwd)/apt-cache"
       echo 'quiet "1";' \
+           'Acquire::Retries "20";' \
            'APT::Install-Recommends "0";' \
            'APT::Install-Suggests "0";' \
-           'APT::Acquire::Retries "20";' \
            'APT::Get::Assume-Yes "true";' \
            'Dpkg::Use-Pty "0";' \
            "Dir::Cache::Archives \"${APT_CACHE_DIR}\"; " \