installer.sh: ensure SOURCE_DONE is accurate

from dateiexplorer:
Another bug with the SOURCE_DONE is also fixed. Previous the local image
was installed instead of the net packages if installation process was
aborted and later continued by the user.

Co-authored-by: dateiexplorer <justus.roederer@dateiexplorer.de>

installer.sh.in

@@ -1301,6 +1301,7 @@ ${BOLD}Do you want to continue?${RESET}" 20 80 || return
     # Create and mount filesystems
     create_filesystems
 
+    SOURCE_DONE="$(get_option SOURCE)"
     # If source not set use defaults.
     if [ "$(get_option SOURCE)" = "local" -o -z "$SOURCE_DONE" ]; then
         copy_rootfs