nixos-install: fail if we can't set up bootloader (#340522)

    ln -sfn /proc/mounts "$mountPoint"/etc/mtab

    export mountPoint

    NIXOS_INSTALL_BOOTLOADER=1 nixos-enter --root "$mountPoint" -c "$(cat <<'EOF'

      set -e

      # Create a bind mount for each of the mount points inside the target file

      # system. This preserves the validity of their absolute paths after changing

      # the root with `nixos-enter`.