Update electron builder

This resolves an issue where the auto-updater on Mac with a standard
(ie. non-admin) user account would leave the app in a broken,
unlaunchable state (although it's not obvious what change in
electron-builder fixes this).

Fixes https://github.com/vector-im/riot-web/issues/8215

package.json

@@ -101,7 +101,7 @@
     "cpx": "^1.3.2",
     "cross-env": "^4.0.0",
     "css-loader": "^2.1.0",
-    "electron-builder": "^20.29.0",
+    "electron-builder": "^20.38.5",
     "electron-builder-squirrel-windows": "^11.6.1",
     "electron-devtools-installer": "^2.2.4",
     "emojione": "^2.2.7",