Optional creation of buildtrees

Background

With #21 (closed) we started caching buildtrees for all elements, whether this is an empty directory or the contents of an actual build from a build element. It has been requested that we should support the creation of them as optional config.

Task description

• Add user config via context to support not caching buildtrees, with the default behaviour left as is
• Handle this config at artifact creation time, implement as needed.
• Ensure that existing test behaviour is not broken, along with new tests for configuration is applied

Acceptance Criteria

Default bst behaviour is not altered, however ensure bst can handle artifacts that are cached as such

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