fix(core-utils): startLogic was called in incorrect context.

[Hill-98]

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Summarize your changes:

Fixed startLogic being called in incorrect context.

[erikian]

Could you elaborate on what's the behavior before / after this change? Is this something that can be tested in some way?

[Hill-98]

The sample code for writing plugins in the documentation currently doesn't work, this change will make it work.

From https://www.electronforge.io/advanced/extending-electron-forge/writing-plugins

export default class MyPlugin extends Pluginbase {
  async startLogic (opts) {
    await this.compileMainProcess();
    return null;
  }

  compileMainProcess () { /* ... */ }
}

newStartFn = plugin.startLogic;: rebind MyPlugin.startLogic function this to undefined.

The official plugin uses all arrow functions, which is fine, but the documentation uses named functions.