It seems like Google has begun to switch their cache results pages over to HTTPS. It seems to be coming on and off for me, but if you search and you see a site that is HTTPS, the Google Cache link may run over HTTPS as well.

Here is a screen shot of a Wikipedia result, since Wikipedia is over HTTPS, showing the Google cache URL to be over HTTPS as well.

I asked Gary Illyes from Google about this and he said Google is ramping up making the cache link over HTTPS:

It makes sense to go HTTPS for HTTPS cached results. So good move Google.

Forum discussion at Twitter.