nixos/default: set configurationRevision

nixos/system/default.nix

@@ -2,6 +2,7 @@
   config,
   lib,
   pkgs,
+  flake,
   ...
 }:
 
@@ -81,6 +82,8 @@ in
       };
     };
 
+    environment.systemPackages = with pkgs; [ nixos-rebuild-ng ];
+
     # Enable firmware-linux-nonfree
     hardware.enableRedistributableFirmware = lib.mkDefault true;
 
@@ -115,7 +118,7 @@ in
       };
     };
 
-    environment.systemPackages = with pkgs; [ nixos-rebuild-ng ];
+    system.configurationRevision = flake.rev or "dirty";
 
     # nixos/modules/misc/version.nix
     users.motd = lib.mkIf cfg.motd.enable ''Welcome to '${config.networking.hostName}' running NixOS ${config.system.nixos.version}!'';