fix: trying to fix backup differences

[Tbaile]

Closes: #1146

[gsanchietti]

Suppress verbose output and gunzip:

Suggested change

	tar -xvf "$BACKUP" -C "$EXTRACTED_BACKUP"
	tar -xzf "$BACKUP" -C "$EXTRACTED_BACKUP" 2>/dev/null

Otherwise this is the raised error:

root@fw:~# sh -x /usr/sbin/send-backup 
+ set -e
+ WORK_DIR=/var/backup
+ date '+%s'
+ BACKUP=/var/backup/backup-1782227752.tar.gz
+ EXTRACTED_BACKUP=/var/backup/extracted
+ MD5=/var/backup/md5
+ MD5_LAST=/etc/backup.md5
+ PASSPHRASE=/etc/backup.pass
+ uci -q get ns-plug.config.system_id
+ SYSTEM_ID=F4B4E624-76A4-4A1C-991A-EEDA77EFFA85
+ uci -q get ns-plug.config.secret
+ SYSTEM_SECRET=dbc658783138b0e8fb30603f2d0fe5ab2aff5572c933651656e569b4c451018f
+ '[' -z F4B4E624-76A4-4A1C-991A-EEDA77EFFA85 ]
+ '[' -z dbc658783138b0e8fb30603f2d0fe5ab2aff5572c933651656e569b4c451018f ]
+ '[' -f /etc/acme/http.header ]
+ mkdir -p /var/backup
+ sysupgrade -k -b /var/backup/backup-1782227752.tar.gz
+ mkdir -p /var/backup/extracted
+ tar -xvf /var/backup/backup-1782227752.tar.gz -C /var/backup/extracted
tar: invalid tar magic