install qgis descriptors into <PREFIX>/share/ on all platforms

Description

Following issue has been affected by this https://gitlab.orfeo-toolbox.org/orfeotoolbox/qgis-otb-plugin/issues/19

Describe what happens and why you think it is a bug

Steps to reproduce

Describe as precisely as possible how to reproduce the bug. Try to isolate a minimal number of steps. Also describe reproducibility (always, random ...).

Configuration information

OS, OTB version or tag, information related to build (binaries, superbuild, system libs ...)