Use QProcessEnvironment::InheritFromParent on X11

Before 885e9acb6e
Application::processStartupEnvironment() returned
QProcessEnvironment::systemEnvironment() which means the current
environment of KWin when this function was called. By setting
InheritFromParent we can replicate this behavior.
This fixes KWin starting processes on X11.

src/main_x11.cpp

@@ -402,6 +402,8 @@ int main(int argc, char *argv[])
     qunsetenv("QT_QPA_PLATFORM");
     qunsetenv("QT_ENABLE_HIGHDPI_SCALING");
 
+    a.setProcessStartupEnvironment(QProcessEnvironment(QProcessEnvironment::InheritFromParent));
+
     KSignalHandler::self()->watchSignal(SIGTERM);
     KSignalHandler::self()->watchSignal(SIGINT);
     KSignalHandler::self()->watchSignal(SIGHUP);