fix: pre-create /nix before nix installer in Docker (no sudo available)

iso/_inner-build-live.sh

@@ -61,7 +61,11 @@ if [[ -n "${NIX_PACKAGES_PREBAKE:-}" ]]; then
     else
         echo "    installing nix (single-user, no-daemon)..."
         rm -rf /nix ~/.nix-profile ~/.nix-defexpr ~/.nix-channels
-        curl -fsSL https://nixos.org/nix/install | sh -s -- --no-daemon --no-channel-add
+        # Pre-create /nix as root so the installer doesn't try 'sudo mkdir'
+        # (sudo isn't installed in this Docker container).
+        mkdir -m 0755 -p /nix
+        curl -fsSL https://nixos.org/nix/install | \
+            NIX_INSTALLER_TRUST_INSTALLER=1 sh -s -- --no-daemon --no-channel-add
         # shellcheck disable=SC1091
         . /root/.nix-profile/etc/profile.d/nix.sh