justfile: fix nom use condition.

turns out just will plumb the stdout of its running processes, so we detect ttys on stderr instead

justfile

@@ -1,6 +1,6 @@
 set positional-arguments # makes variables accesible as $1 $2 $@
 export GUM_FILTER_HEIGHT := "15"
-nom := `if [[ -t 1 ]] && command -v nom >/dev/null; then echo nom; else echo nix; fi`
+nom := `if [[ -t 2 ]] && command -v nom >/dev/null; then echo nom; else echo nix; fi`
 nix_eval_opts := "--log-format raw --option warn-dirty false"
 
 @_default:
@@ -26,7 +26,7 @@ run-vm machine=`just _a_machine` *_:
     | xargs -d'\n' nix flake update "$@"
 
 @repl $machine=`just _a_machine` *_:
-  set -v; nixos-rebuild --flake .#"$machine" repl "${@:2}"
+  set -v; NIX_NO_NOM=1 nixos-rebuild --flake .#"$machine" repl "${@:2}"
 
 @eval $machine=`just _a_machine` $attrpath="system.build.toplevel.outPath" *_:
   set -v; nix eval {{nix_eval_opts}} ".#nixosConfigurations.\"$machine\".config.$attrpath" --show-trace "${@:3}"

shell.nix

@@ -1,6 +1,18 @@
 { pkgs ? import <nixpkgs> {} }:
+
+let
+  nixos-rebuild-nom = pkgs.writeScriptBin "nixos-rebuild" ''
+    if [[ -t 1 && -z "''${NIX_NO_NOM-}" ]]; then
+      exec ${pkgs.lib.getExe pkgs.nixos-rebuild} -L "$@" |& ${pkgs.lib.getExe pkgs.nix-output-monitor}
+    else
+      exec ${pkgs.lib.getExe pkgs.nixos-rebuild} -L "$@"
+    fi
+  '';
+in
+
 pkgs.mkShellNoCC {
   packages = with pkgs; [
+    nixos-rebuild-nom
     just
     jq
     gum