arch_boxes_sync: Fix old releases cleanup not working due to NUL character

Kristian Klausenrequested to merge
klausenbusk/infrastructure:arch-boxes-sync-fix into master

--output-delimiter="" apparently means use the NUL character[1].

[1] https://git.savannah.gnu.org/cgit/coreutils.git/commit/?id=0e46753d7c9519d7378cd3a4e0951a36ac32ffe7

Merge request reports