fix: restore just switch on current nixpkgs

nixos/quickshell.nix

@@ -2,13 +2,19 @@
 makeEnable config "myModules.quickshell" false {
   home-manager.sharedModules = [
     inputs.caelestia-shell.homeManagerModules.default
-    ({ lib, nixos, ... }:
+    ({ lib, nixos, pkgs, ... }:
       let
         hyprlandEnabled = lib.attrByPath ["myModules" "hyprland" "enable"] false nixos.config;
+        system = pkgs.stdenv.hostPlatform.system;
+        caelestiaPackage = inputs.caelestia-shell.packages.${system}.with-cli.override {
+          # Caelestia pins app2unit 1.0.3, which no longer builds on this nixpkgs snapshot.
+          app2unit = pkgs.app2unit;
+        };
       in
       {
         programs.caelestia = {
           enable = true;
+          package = caelestiaPackage;
           cli.enable = true;
           systemd.target = lib.mkDefault (
             if hyprlandEnabled