Pass cache imports from solve ptions to solve request

[Ignas]

At the moment build --import-cache option is ignored when not using any frontend. This makes sure the option is passed along to the actual solve request.

[tonistiigi]

I'm not sure if it is intentional that this is not working automatically. The cache source from the initial request could be always included, unless there is some use case where frontend absolutely wants to ignore them.

[jedevc]

Ah, I had also noticed this at some point 🎉 Thanks for putting a patch together!