You don’t need to make any entry in rc.local or symlink - Qubes has already covered
this as you will see if you examine /rw/bind-dirs/var/spool/cron, and
cron will work out of the box when you enable the service.
You should use crontab -e to create and change crontab entries.
I see your entry:
* * * * * user /rw/home/user/path/to/./backup.sh
Can you try that without the user field?
It would be helpful if you posted the contents of backup.sh