The GetBuilds function, available on both HCL2 and legacy JSON
configuration objects, used to return the Build interface.
This typing by interface is not useful in this instance, since all the
uses of `GetBuilds' are self-contained within Packer, and we're never
using any other implementation for it than `*CoreBuild`.
We've been relying on the dynamic type for all the builds being
*CoreBuild in some places of the code, so to avoid potential surprises
in the future, we'll change the signature now so it returns only
concrete types.
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