nixos-rebuild: always set flakeFlags

Otherwise a rebuild can fail when a flake is autodetected
in /etc/nixos/flake.nix and the system doesn't have flakes
globally enabled.

(cherry picked from commit 28dae620b2)

pkgs/os-specific/linux/nixos-rebuild/nixos-rebuild.sh

@@ -20,7 +20,7 @@ origArgs=("$@")
 copyClosureFlags=()
 extraBuildFlags=()
 lockFlags=()
-flakeFlags=()
+flakeFlags=(--extra-experimental-features 'nix-command flakes')
 action=
 buildNix=1
 fast=
@@ -118,7 +118,6 @@ while [ "$#" -gt 0 ]; do
         ;;
       --flake)
         flake="$1"
-        flakeFlags=(--extra-experimental-features 'nix-command flakes')
         shift 1
         ;;
       --no-flake)