cubicweb #17132990 legacy cubes not detected in not-activated virtualenv [validation pending]

From cubicweb 3.26, running venv/bin/cubicweb list cubes fails to load legacy cubes (i.e. non-package cubes). It's not clear why this suddenly happen, perhaps this is a side effect of packaging cleanups undertaken during 3.26 development.

done in3.26.1
load left0.000
closed by#9d08f89a3d85 [cwconfig] Always use "sys.prefix" to determine installation prefix