[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.
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html