StudioWelcome: Fix QTC_ASSERT

Deleting QtcProcess instance directly from
one of its signal handlers will lead to crash!

I cannot confirm this crash actually happened, but we should play save
and delete the archive from the event loop.

Change-Id: Ie27291c9ac8b300478ed8396d25a76b343f238be
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/studiowelcome/examplecheckout.cpp

@@ -424,7 +424,7 @@ void FileExtractor::extract()
     });
 
     QObject::connect(archive, &Utils::Archive::finished, this, [this, archive](bool ret) {
-        delete archive;
+        archive->deleteLater();
         m_finished = ret;
         m_timer.stop();