buildx(build): resolveCacheToAttrs func

[crazy-max]

follow-up moby/buildkit#4788 (review)

When using gha cache export type, set repository and ghtoken attributes.

Needs follow-up on build-push-action and bake-action to use this func.

[crazy-max]

For bake-action that's tricky. We need to first load the definition to check cache-to usage in targets and would then need to use overrides to correct them.