add explaining comments in script files

scripts/build_nix_cache.sh

+# To save the nix cache in a file, for future use
+# (usefull when working in Grid'5000)
 nix-store --query --references $(nix-instantiate default.nix) | \
     xargs nix-store --realise | \
     xargs nix-store --query --requisites | \

scripts/install_nix.sh

@@ -4,4 +4,8 @@ sudo-g5k
 sudo su root --command "echo 1 > /proc/sys/kernel/unprivileged_userns_clone"
 curl https://nixos.org/releases/nix/nix-2.6.0/install | sh
 source ${HOME}/.nix-profile/etc/profile.d/nix.sh
-nix-store --import < cache_nix
+
+# Uncomment the line below if you want to import the nix cache from a file
+# (useful when working in Grid'5000)
+
+# nix-store --import < cache_nix