fix(install): offline installs copy the ISO store into the target up front

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>

pkgs/nomarchy-install/nomarchy-install.sh

@@ -437,6 +437,18 @@ else
     warn "flake archive failed — first rebuild will need network."
 fi
 
+# Offline: sidestep substituter plumbing entirely — make every ISO store
+# path valid in the target store up front. nixos-install's in-target
+# build then finds all build tools locally and only the per-machine
+# config derivations are built. (Two earlier attempts to route this
+# through substituters — env config and the forwarded --substituters
+# flag — still left the plan building gcc from source.)
+if [[ ${#NIXOS_INSTALL_OPTS[@]} -gt 0 ]]; then
+    info "Copying the ISO store into the target (offline install)..."
+    nix --extra-experimental-features nix-command \
+        copy --all --no-check-sigs --to "local?root=/mnt"
+fi
+
 nixos-install --no-root-passwd "${NIXOS_INSTALL_OPTS[@]}" --flake "$FLAKE_DIR#default"
 success "System installed (bootloader in place)"