nixos-rebuild: Update all channels of user root.

Should make it even easier to use custom channels, because whenever the
user does a "nixos-rebuild --upgrade", it will also upgrade possibly
used ("used" as in referenced in configuration.nix) channels besides
"nixos". And if you also ship a channel tied to a particular version of
nixpkgs or even remove the "nixos" channels, you won't run into
unexpected situations where the system is not updating your custom
channels.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

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

@@ -93,9 +93,9 @@ if [ "$action" = switch -o "$action" = boot -o "$action" = test ]; then
 fi
 
 
-# If ‘--upgrade’ is given, run ‘nix-channel --update nixos’.
+# If ‘--upgrade’ is given, run ‘nix-channel --update’.
 if [ -n "$upgrade" -a -z "$_NIXOS_REBUILD_REEXEC" ]; then
-    nix-channel --update nixos
+    nix-channel --update
 fi
 
 # Make sure that we use the Nix package we depend on, not something