installer.sh: workaround for sandbox builds

The new default nix.useSandbox = true breaks calling nix-env from
nixos-enter with the error

error: cloning builder process: Operation not permitted

Until a proper fix is found, we explicitely disable sandbox builds
in the installer script when generating the final configuration
on the install target.

modules/installer.sh

@@ -169,8 +169,11 @@ nixos-generate-config --root /mnt
 export NIX_PATH=/nix/var/nix/profiles/per-user/root/channels/nixos:nixos-config=/etc/nixos/configuration.nix
 
 informOk "generating system configuration..."
+## FIXME: starting with 18.09, nix.useSandbox defaults to true, which breaks the execution of
+## nix-env in a chroot when the builder needs to be invoked.  Disabling the sandbox
+## is a workaround.
 nixos-enter --root /mnt -c "mv /resolv.conf /etc && \
-  /run/current-system/sw/bin/nix-env -p /nix/var/nix/profiles/system -f '<nixpkgs/nixos>' --set -A system"
+  /run/current-system/sw/bin/nix-env --option sandbox false -p /nix/var/nix/profiles/system -f '<nixpkgs/nixos>' --set -A system"
 informOk "...system configuration done"
 
 informOk "activating final configuration..."