gn-native from qtwebengine conflicts with the one from meta-chromium

meta-qt6 has a recipe for generate-ninja (gn-native) that's built out of the sources of qtwebengine. meta-chromium exposes a separate recipe for gn-native that builds upstream chromium gn sources. meta-chromium layer has a higher priority so if both it and meta-qt6 are used, the latter recipe for gn will be selected. qtwebengine cannot be built using upstream gn as cmake/FindGn.cmake checks the exact version and requires it to come from qtwebengine. This creates a conflict if one wants to build both chromium and qtwebengine for the same image (or if we even just add meta-chromium to BBLAYERS for that matter).

How to reproduce:

1. Add meta-qt6 and meta-chromium to BBLAYERS.
2. bitbake qtwebengine
3. Observe do_configure() failure due to missing gn