Make Program Updater choose the same build for download

We're probably stuck offering the duo of RC_1_2 and RC_2_0 for some
time in the future. So hardcode the choices and make the Program Updater
choose the variant the user currently uses.

src/gui/programupdater.cpp

@@ -97,11 +97,23 @@ void ProgramUpdater::rssDownloadFinished(const Net::DownloadResult &result)
             : QString {};
     };
 
+#ifdef QBT_USES_LIBTORRENT2
+
+#ifdef Q_OS_MACOS
+    const QString OS_TYPE = u"Mac OS X (lt20)"_s;
+#elif defined(Q_OS_WIN)
+    const QString OS_TYPE = u"Windows x64 (lt20)"_s;
+#endif // Q_OS_MACOS
+
+#else // QBT_USES_LIBTORRENT2
+
 #ifdef Q_OS_MACOS
     const QString OS_TYPE = u"Mac OS X"_s;
 #elif defined(Q_OS_WIN)
     const QString OS_TYPE = u"Windows x64"_s;
-#endif
+#endif // Q_OS_MACOS
+
+#endif // QBT_USES_LIBTORRENT2
 
     bool inItem = false;
     QString version;