FS-8271 If using system apt repo list, then include the supplementary ones too.

debian/util.sh

@@ -289,6 +289,14 @@ build_debs () {
       esac
     done
     shift $(($OPTIND-1))
+    if [ "$custom_sources_file" == "/etc/apt/sources.list" ]; then
+        # If you are using the system sources, then it is reasonable that you expect to use all of the supplementary repos too
+        cat /etc/apt/sources.list > /tmp/fs.sources.list
+        for X in /etc/apt/sources.list.d/*; do cat $X >> /tmp/fs.sources.list; done
+        custom_sources_file="/tmp/fs.sources.list"
+        apt-key exportall > "/tmp/fs.asc"
+        custom_keyring="/tmp/fs.asc"
+    fi
     if [ "$custom_sources_file" == "" ]; then
         # Caller has explicitly set the custom sources file to empty string. They must intend to not use additional mirrors.
         use_custom_sources=false