You're assuming the version of electrum you downloaded from the software center is the same as you would have compiled yourself. Why risk it? Just download the tools to compile your own copy.
Also, go through your firewall rules to make certain that nothing on your machine can connect out, or connect in. Of course, then you have an internet connected machine not receiving software updates...
REally, at the end - if you want cold storage, you need to create an airgapped machine, there's no point in doing it halfway.