electron-builder: Disable .zip build target on macOS

The macOS .zip build target for electron-builder is very computationally
intensive (seemingly due to heavy use of 7-zip?) and timing out for us
in CI. So... disable the .zip build target on macOS entirely.

Note: The .dmg and .zip targets are the default targets enabled by
electron-builder on macOS. (See: https://www.electron.build/cli#target)
So, by explicitly specifying only the .dmg target,
we can effectively disable the .zip target.

script/electron-builder.js

@@ -197,6 +197,9 @@ let options = {
         { "CFBundleURLName": "Atom Shared Session Protocol" }
       ]
     },
+    "target": [
+      { "target": "dmg" }
+    ],
   },
   "dmg": {
     "sign": false,