[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.

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^build$
^dist$
\.pyc$
\.pyo$
\.bak$
\.old$
\~$
\#.*?\#$
\.swp$
^doc/book/en/apidoc$