Handle non-existent :path

[SevereOverfl0w]

See #109

pack.alpha/src/juxt/pack/jib.clj

Line 114 in 47ee078

(when (Files/exists path (into-array LinkOption []))

is the likely culprit.

The question is, what should be done? Presumably the Clojure CLI ignores this, and we should do the same.

We may want to move the exists check into :builder to match the exact behaviour of the CLI (I'm making assumptions here about what the CLI ends up doing!)

:builder (if (Files/exists ...) (.addEntryRecursive builder ...) builder)