Hyprland reaps its own children, so os.execute() gets ECHILD from waitpid and
never sees an exit status. Every call reported failure, which meant the NVIDIA
env detection in hypr/nvidia.lua never set NVD_BACKEND, LIBVA_DRIVER_NAME or
__GLX_VENDOR_LIBRARY_NAME. Read a marker off stdout instead.
Closes#6914
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Evaluate NVIDIA session env vars from default/hypr/nvidia.lua during Hyprland startup instead of appending them to each user's envs.lua during setup. This removes the per-user NVIDIA setup script execution while preserving the GSP/non-GSP detection.
