apt-mirror: use tagged APT snapshots when building a Stretch release from a tag.

auto/scripts/apt-mirror

@@ -33,7 +33,8 @@ CURRENT_BRANCH=$(current_branch)
 
 if [ "$BASE_BRANCH" = stable ] \
        || [ "$BASE_BRANCH" = testing ] \
-       || [ "$CURRENT_BRANCH" = feature/stretch ]
+       || [ "$CURRENT_BRANCH" = feature/stretch ] \
+       || ( on_a_tag && [ "$BASE_BRANCH" = feature/stretch ] ) \
 then
     case "$ARCHIVE" in
        debian-security)