[cw-ctl] use default `mkdtemp()` behaviour to store backup file internally
When using `mkdtemp(dir=instbkdir)`, a dedicated system user which hasn't the
permissions to write in /var/lib/cubicweb/instances/$INSTANCE/backup will not
be able to run the db-dump command.
addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile