Common configuration "icon" is ignored for macOS

Docs: https://www.electron.build/configuration#icon

This is broken only for macOS.  
Windows and Linux builds are ok.

Works:
  - `26.1.0`  

Fail:
  - `26.2.0`
  - `26.3.0`

Config broken:
```js
{
  // ...
  icon: 'test/icon.icon',
  mac: {
    // ...
  },
  // ...
}
```

Config for `26.2.0`+:
```js
{
  // ...
  icon: 'balablabla',
  mac: {
     icon: 'test/icon.icon',
  },
  // ...
}
```

