Fix the two ShellCheck warnings and hold CI at that level

The workflow went in at severity: error on the assumption that a
never-linted repository would have a backlog worth grandfathering. It
did not -- error found nothing, and warning found exactly two things, so
the cautious setting was protecting against a problem that was not
there.

zimbra_backup.sh: SC2024, sudo does not affect redirects. The
`> "$BACKUP_FILE"` runs as root rather than as the sudo'd zimbra user,
so backups landed root-owned inside a directory the script deliberately
chowns to zimbra:zimbra. Kept the redirect -- root can always write
there, and piping into `tee` would put tee's status in $? and hide a
zmmailbox failure -- and handed ownership over explicitly afterwards.
The suppression is narrow and states why.

disk_cleanup.sh: SC2034, total_freed was assigned and never read.

CI now holds at warning with a clean tree, so anything that trips it is
new rather than inherited.

disk_cleanup.sh

@@ -208,8 +208,6 @@ clean_package_caches() {
 
 # Clean temporary files
 clean_temporary_files() {
-  local total_freed=0
-  
   for dir in "${DIRS_TO_CLEAN[@]}"; do
     if [ ! -d "$dir" ]; then
       log "Warning: Directory $dir does not exist, skipping..."