nixos/nixos-install: Unset system

The system variable is used from the (possibly polluted) shell
environment.
This causes nixos-install to fail in a nix-shell because the system
shell variable is automatically set to the current system (e.g.
x86_64-linux).

nixos/modules/installer/tools/nixos-install.sh

@@ -13,6 +13,7 @@ extraBuildFlags=()
 
 mountPoint=/mnt
 channelPath=
+system=
 
 while [ "$#" -gt 0 ]; do
     i="$1"; shift 1