Add '-externalupdater' command-line argument.

Now no-autoupdater mode can be switched on in runtime.
Also TDESKTOP_DISABLE_AUTOUPDATE build is disabled in CI (trivial).

Fixes #4895.

Telegram/SourceFiles/application.h

@@ -61,9 +61,7 @@ private:
 	void singleInstanceChecked();
 
 private:
-#ifndef TDESKTOP_DISABLE_AUTOUPDATE
 	std::unique_ptr<Core::UpdateChecker> _updateChecker;
-#endif // !TDESKTOP_DISABLE_AUTOUPDATE
 
 };