On Redox there is no udev-based DRM device enumeration. KWin's DRM
backend relies on m_udev->listGPUs() which returns nothing without udev.
Add a fallback: when no KWIN_DRM_DEVICES is set and the session is
kde-wayland, inject KWIN_DRM_DEVICES=/scheme/drm/card0 so KWin knows
which device to open. This fixes the 'No suitable DRM devices' error
that prevented KWin from starting on Redox.
Added test: build_environment_sets_kwin_drm_devices_default_for_kde_wayland
to verify the fallback is applied correctly.
vasilito