[web/staticcontrollers] create a unique temporary file for concat handling
Predictable names means different processes can race against each other.
Let's avoid that and use mkstemp to get a real unique temporary file,
and atomically rename it when we're done.
Closes #3524182
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