ci/eval: fix local full eval

The change to use `builtins.storePath` was good - for when the store
path *is* already part of the nix store. In all my tests so far, that
was already the case, because I was iterating on the solution and the
Eval results stayed the same.

But when this is run on a entirely new commit, these the values for
`afterDir` and `combinedDir` are *not* in the store, yet. As part of
running `eval.full` on a new commit they will be created. `eval.full` is
linked up, so that the values passed around there will actually be
derivations, which might not be realized, yet.

Checking whether the input is a path or not fixes this for both cases.

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