[autotest/wayland] Force kscreenlocker_greet to QMLSCENE_DEVICE softwarecontext

OpenGL/EGL doesn't work on build.kde.org. Because of that the
kscreenlocker_greet aborts on startup and we cannot really test whether
it works on build.kde.org. E.g. we never actually see the window.

This change just makes sure that QMLSCENE_DEVICE=softwarecontext is
passed to the greeter in the LockScreenTest. Hopefully that fixes it
on build.kde.org.

autotests/wayland/lockscreen.cpp

@@ -206,6 +206,7 @@ void LockScreenTest::initTestCase()
     QCOMPARE(screens()->geometry(0), QRect(0, 0, 1280, 1024));
     QCOMPARE(screens()->geometry(1), QRect(1280, 0, 1280, 1024));
     setenv("QT_QPA_PLATFORM", "wayland", true);
+    setenv("QMLSCENE_DEVICE", "softwarecontext", true);
     waylandServer()->initWorkspace();
 }