builderv2: do package init-cache automatically

[marmarek]

How to file a helpful issue

The problem you're addressing (if any)

Normally builder creates the whole build env from scratch each time. Twice for a given package ("prep" and "build" stages). That takes time.

The solution you'd like

Automatically do init-cache stage if cache isn't there already. Skip the automatic init-cache, if it's created already. Do not skip it, if user explicitly requested to re-create the cache. The re-creating can require extra option, but the message about skipping init-cache should include info how to force re-create.

The value to a user, and who that user might be

Faster build times.

Completion criteria checklist

(This section is for developer use only. Please do not modify it.)

[marmarek]

For similar UX reasons, maybe it should not remove old cache if init-cache fails?