bake: set attribute even if diagnosed as duplicated

[crazy-max]

Carry changes from #1025 related to an issue with hcl attributes not set if diagnosed as duplicated: https://github.com/docker/buildx/pull/1025#discussion_r843336438.

Needs to fork the merge logic from hcl repo. It might be better if hcl exposed its mergedbodies interface. Don't think we should fork the repo and vendor it here with this small change though. Maybe there is a better way?