arch_boxes_sync: Quick service filesystem hardening

Should protect the filesystem from the script going amok and nuking the
filesystem (shouldn't happen).

roles/arch_boxes_sync/files/arch-boxes-sync.service

@@ -4,3 +4,6 @@ Description=Sync arch-boxes releases
 [Service]
 Type=oneshot
 ExecStart=/usr/local/bin/arch-boxes-sync
+ProtectSystem=strict
+PrivateTmp=true
+ReadWritePaths=/srv/ftp/images