fix(casks): use postflight_steps and rewrite GoReleaser output on push

GoReleaser writes the quarantine hook as a postflight block on each release.
Homebrew 7 deprecates that block. The new workflow rewrites it to
postflight_steps after every push to main.

Casks/harness-sync.rb

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