fix: set NIX_CONFIG build-users-group empty to skip nixbld group check in Docker

iso/_inner-build-live.sh

@@ -64,6 +64,8 @@ if [[ -n "${NIX_PACKAGES_PREBAKE:-}" ]]; then
         # 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
+        # Disable build-users-group so nix doesn't complain about missing nixbld group.
+        export NIX_CONFIG="build-users-group = "
         curl -fsSL https://nixos.org/nix/install | \
             NIX_INSTALLER_TRUST_INSTALLER=1 sh -s -- --no-daemon --no-channel-add
         # shellcheck disable=SC1091