AppImage build doesn't include images

[mykter]

After specifying this build config:

    "directories": {
      "buildResources": "resources"
    }

electron-builder won't include the resources directory in the asar, which means the images can't be used for systray creation.

But removing that entry (which should now be possible in 20.0.4) bumps into this bug: electron-userland/electron-builder#2599

Could work around it by adding a files directive, but hopefully once 2599 is fixed everything will just work with the default config.