The local registry is a more future-proof way to prevent cargo from
accessing the network during the build. Unfortunately, this is only
used during the build of cargo-native for now. The snapshot used while
building cargo-native supports using a local registry ("source
replacement") however the version of cargo supported by rust 1.10 does
not.
Until we can build a new-enough version of cargo that supports
source-replacement, we'll have to patch cargo directly to prevent it
from accessing the network. Once we do have a new-enough cargo, we can
stop populating cargo_home/registry and only create cargo_registry.
