Revert "Temporarily disable APT proxy support"

This reverts commit 47dc8a09.

.gitlab-ci.yml

@@ -21,6 +21,7 @@ variables:
       --passwords=${TEST_DIR}/passwords.yml
       --num-hosts=1
       ${LIBVIRT:+-e libvirt.remote_host=${LIBVIRT#*@} -e libvirt.remote_user=${LIBVIRT%@*}}
+      ${APT_PROXY:+-e config.apt_proxy=${APT_PROXY}}
       $CREATE_ENV_VARS $BUILD_DIR
 
     - with-ssh-key floatup ${LIBVIRT:+--ssh $LIBVIRT} --inventory $BUILD_DIR/hosts.yml --ram 2048 --cpu 2 --image ${VM_IMAGE:-bookworm} ${FLOATUP_ARGS} up