Write the cask with postflight_steps and depends_on :macos

Matches what Homebrew 7's style check wants; Ruby flight blocks are
only tolerated in third-party taps for now.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitea/workflows/desktop.yml

@@ -253,11 +253,12 @@ jobs:
             desc "Desktop app wrapping the Navidrome music server"
             homepage "$FORGE_URL/$GITHUB_REPOSITORY"
 
+            depends_on :macos
+
             app "Quintodrome.app"
 
-            postflight do
-              system_command "/usr/bin/xattr",
-                             args: ["-dr", "com.apple.quarantine", "#{appdir}/Quintodrome.app"]
+            postflight_steps do
+              run "/usr/bin/xattr", args: ["-dr", "com.apple.quarantine", "{{appdir}}/Quintodrome.app"]
             end
 
             zap trash: [