[cwconfig] Do not override user-defined CW_MODE in virtualenv setup
And thus consider `_forced_mode` first.
Related to #14789440.
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/html/index.html
Files: /usr/share/doc/cubicweb-documentation/html/*