Fix fastlane mac beta build for multiplatform scheme

Without sdk:"macosx", gym treats the multiplatform scheme as iOS and
fails with "IPA invalid". Without an explicit destination xcodebuild
picked tvOS. The output_name also needed the .app suffix removed since
gym appends .pkg for macOS app-store exports.

fastlane/Fastfile

@@ -266,8 +266,10 @@ platform :mac do
 
     build_mac_app(
       scheme: SCHEME,
+      sdk: "macosx",
+      destination: "generic/platform=macOS",
       output_directory: "fastlane/builds/#{version}-#{build}/macOS",
-      output_name: "#{APP_NAME}-#{version}-macOS.app",
+      output_name: "#{APP_NAME}-#{version}-macOS",
       export_method: "app-store",
       export_options: {
         provisioningProfiles: {