nixos-rebuild: make 'edit' work with directories

$NIXOS_CONFIG can be set to a directory, in which case the file used
is $NIXOS_CONFIG/default.nix. This updates 'nixos-rebuild edit' to
handle that case correctly.

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

@@ -288,6 +288,9 @@ fi
 if [ "$action" = edit ]; then
     if [[ -z $flake ]]; then
         NIXOS_CONFIG=${NIXOS_CONFIG:-$(nix-instantiate --find-file nixos-config)}
+        if [[ -d $NIXOS_CONFIG ]]; then
+            NIXOS_CONFIG=$NIXOS_CONFIG/default.nix
+        fi
         exec "${EDITOR:-nano}" "$NIXOS_CONFIG"
     else
         exec nix edit "${lockFlags[@]}" -- "$flake#$flakeAttr"