By default "mktemp" creates a temporary file in the current working directory,
which in the context of Debian's cron is /. To avoid littering /, we add the
option "--tmpdir" that use $TMPDIR or /tmp instead.
The function "build_rsync_main_cmd" is called in a subshell, so it can't
effectively modify it's parent variable "temp_files". To correctly cleanup
those temporary files, we do it when this specific function exits.