Substituter plumbing through nixos-install proved unreliable (env config
is overridden by its hardcoded flags; the forwarded --substituters flag
still left the plan building gcc from source). Make every ISO path valid
in the target store with one nix copy --all before nixos-install — the
in-target build then resolves everything locally and only the per-machine
config drvs are built.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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