ci(macos): add SPM cache and increase parallel builds

Author: MichaelVerdon

tests/macos/Runner/AppDelegate.swift

@@ -1,9 +1,13 @@
 import Cocoa
 import FlutterMacOS
 
-@NSApplicationMain
+@main
 class AppDelegate: FlutterAppDelegate {
   override func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
     return true
   }
+
+  override func applicationSupportsSecureRestorableState(_ app: NSApplication) -> Bool {
+    return true
+  }
 }