fix(casks): rewrite postflight to postflight_steps

Casks/kportal.rb

@@ -33,10 +33,9 @@ cask "kportal" do
 
   binary "kportal"
 
-  postflight do
-    if OS.mac?
-      system_command "/usr/bin/xattr",
-                     args: ["-dr", "com.apple.quarantine", "#{staged_path}/kportal"]
+  postflight_steps do
+    on_macos do
+      run "/usr/bin/xattr", args: ["-dr", "com.apple.quarantine", "{{staged_path}}/kportal"]
     end
   end