The initial fix only wrote the prefix npmrc on a fresh Node install, so
pre-existing bundled-Node installs (Node already present) were not repaired
by re-running the installer — install_node/ensure_node skip when Node is
already up to date.
Extract the redirect into an idempotent helper
(configure_managed_node_npm_prefix / _nb_configure_npm_prefix) that no-ops
when there's no Hermes-managed npm, and call it unconditionally from
check_node (install.sh) and at the top of ensure_node (node-bootstrap.sh).
Re-running the install command now repairs an affected install in place,
not just brand-new ones.
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