Rework cache key to be simpler

[wolfv]

I think for the cache key we can just use the serialized / rendered cache output to JSON to capture all Jinja variables. We should also find what variants are applied to dependencies and lastly, the build / target-platform should influence the cache key.

Currently, "used variables" from Jinja are ignored in the cache section.
Also, the script section might contain "lazily" evaluated Jinja variables.