[merge] Simple correction to KernelLikeTreeCreator

[John Arbash Meinel]

When I introduced the ability to cache intermediate trees, I missed the
fact that _create_tree might actually get called when the target doesn't
exist yet.

The attached patch just adds a mkdir() call, so that new caches can be
created. It still is the case that the base cache directory must exist.

John
=:->
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