disable pkg caching in context bundles

[nerdvegas]

It doesn't make sense to cache packages when they're present in a context bundle (the variant payloads are already effectively localized). Update so that caching is always disabled in bundles.

[JeanChristopheMorinPerso]

What if the payload lived on a network? For example, if rez-bundle was to be used to ship an environment to another studio, it's possible that it'll be stored on a network sot that the artists can all use the same bundle.