fix(backup_download): no log on stdout when using '--output -' flag

[EtienneM]

╰─$ go build -o scalingo-cli ./scalingo && ./scalingo-cli -a my-app backups-download --addon ad-c98ed74e-fbe6-4154-9806-e957cb9304e7 --output - > b
-----> Selected the most recent backup
 305 B / 305 B [======================================//===========================================] 100.00% 0s
╭─etiennem@biniou ~/Documents/Scalingo/golang/src/github.com/Scalingo/cli ‹2.7.2› ‹fix/645/backup_dl_output*› 
╰─$ head b                                                                                                                                         
?N0p/G#Kγn
           
$.p/ -Pa.ˁĀA,b֕+x
                l~ .(xI+;EaB5q,+?0l>qW>
kinr{6pnN&WEdMƢC(h"CgHYZ!EYgwU  zEfj|w0ZVx

Fix #645

• Add a changelog entry in the section "To Be Released" of CHANGELOG.md