cicero image: enable sandboxing for nix builds

Some nix builds use global files such as /var/log/
that the nixbld users don't have access to.
Although these should ideally be fixed, we still
want to be able to build them until they are.

see NixOS/nixpkgs#180734

Author: dermetfan

nix/cloud/oci-images.nix

@@ -32,7 +32,6 @@ in {
     nixConf = runCommandNoCC "nix.conf" {} ''
       mkdir -p $out/etc/nix
       cat > $out/etc/nix/nix.conf <<'EOF'
-      sandbox = false
       experimental-features = nix-command flakes
       EOF
     '';