Pithos desktop: no electron-builder publishing

On a CI runner electron-builder assumes a GitHub release and writes
auto-update files for it; with no repository it then crashed after the
Linux packages were built. Pithos ships through its own channels
(dl.silentmode.st and the release manifest), so publishing is off.

package.json

@@ -107,6 +107,7 @@
     },
     "dmg": {
       "title": "Pithos ${version}"
-    }
+    },
+    "publish": null
   }
 }