Fixes#356. Instead of downloading a node binary with our closure
serialization code linked-in, this PR instead publishes the
`nativeruntime.node` produced by the NodeJS SDK build as part of the SDK.
This has a number of advantages. First, it is vastly more easy to
develop closure.cc in this configuration. Second, we have the ability
to ship different `nativeruntime.node`s side-by-side, paving the way
for enabling future versions of Node. Third, we don't have to stay
in the business of shipping custom builds of Node, although we do still
need to ship a version of Node with minor modifications in order for
Windows to still work.
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