Add mergeASARs = false to the mac configuration

See docs at
https://www.electron.build/configuration/mac#:~:text=mergeASARs%20%3D%20true,for%20%E2%80%9Cuniversal%E2%80%9D%20arch.
and discussion at
electron-userland/electron-builder#6735

Apparently, the option needs to be disabled at this time when building
for universal Mac.

Author: mokagio

electron-builder.json

@@ -23,7 +23,8 @@
         "target": "zip",
         "arch": ["universal"]
       }
-    ]
+    ],
+    "mergeASARs": false
   },
   "dmg": {
     "title": "Simplenote Installer",