use preBuild instead

appview/pages/pages.go

···
 			return pairs, nil
 		},
 		"append": func(s []string, values ...string) []string {
-			for _, v := range values {
-				s = append(s, v)
-			}
+			s = append(s, values...)
 			return s
 		},
 		"timeFmt": humanize.Time,

flake.nix

···
             pname = "appview";
             version = "0.1.0";
             src = gitignoreSource ./.;
-            postConfigureHook = ''
+            preBuild = ''
               cp -f ${htmx-src} appview/pages/static/htmx.min.js
               cp -f ${lucide-src} appview/pages/static/lucide.min.js
               ${pkgs.tailwindcss}/bin/tailwindcss -i input.css -o appview/pages/static/tw.css