nixos-rebuild: Prefer the module lib when available

pkgs.lib and lib need not match, so prefer the one that the modules get.

For example:

```nix
nix-repl> :lf nixpkgs
nix-repl> (lib.nixosSystem { modules = [ ({ lib, ... }: assert lib?nixosSystem; { nixpkgs.hostPlatform = "x86_64-linux"; }) ]; }).pkgs.lib?nixosSystem
false
```

The code is backwards compatible, for the case where you evaluate a
config whose nixpkgs doesn't have the preceding commit yet.

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

@@ -567,7 +567,7 @@ if [ "$action" = repl ]; then
                 configuration._module.specialArgs //
                 {
                   inherit (configuration) config options;
-                  inherit (configuration.pkgs) lib;
+                  lib = configuration.lib or configuration.pkgs.lib;
                   inherit flake;
                 };
           in builtins.seq scope builtins.trace motd scope