diff --git a/nix-os/unstable-packages.nix b/nix-os/unstable-packages.nix
index 67f0f5b..27e0727 100644
--- a/nix-os/unstable-packages.nix
+++ b/nix-os/unstable-packages.nix
@@ -31,6 +31,7 @@ in
     };
   };
   config._module.args.unstablePkgs = config.unstable.pkgs;
-  # FIXME: move it to the system derivation output (overrideAttrs config.system.build.toplevel?)
-  config.environment.etc."NIXOS-UNSTABLE-VERSION".text = nixos-unstable-version;
+  config.system.extraSystemBuilderCmds = lib.mkIf config.unstable.enable ''
+    echo ${nixos-unstable-version} > $out/nixos-unstable-version
+  '';
 }
\ No newline at end of file