nix: necessary line for g5k

scripts/install_nix.sh

 #!/usr/bin/env bash
 cd ~/expe_g5k
 sudo-g5k
+mkdir -m 0755 -p /nix/var/nix/{profiles,gcroots}/per-user/$USER
 # sudo su root --command "echo 1 > /proc/sys/kernel/unprivileged_userns_clone"
 # curl https://nixos.org/releases/nix/nix-2.13.3/install | sh
 sh <(curl -L https://nixos.org/nix/install) --no-daemon --yes