Fix: Launcher: wait for Blender to exit
If blender-launcher is called with backgorund command line flag,
wait for the blender process to exit and return its return value

Unity uses Blender in background mode to convert .blend to .fbx
Since Blender 3.0 registers blender-launcher.exe as the default
file handler for .blend files, Unity uses the launcher instead of
Blender itself.
When the blender-launcher.exe exits before the resulting fbx file
is created, Unity fails to import or even worse imports the .fbx
from a previous run.