AppCheck Build Error During Prerender for Angular SSR

### Version info

**Angular:** 13.0.3

**Firebase:** 9.6.4

**AngularFire:** 7.2.0

**Other (e.g. Ionic/Cordova, Node, browser, operating system):** Node 14

### How to reproduce these conditions
**Steps to set up and reproduce**

After setting up AppCheck as such:

```typescript

import { initializeAppCheck, provideAppCheck, ReCaptchaV3Provider } from '@angular/fire/app-check';

provideAppCheck(() => initializeAppCheck(getApp(), { provider: new ReCaptchaV3Provider(environment.recaptchaSiteKey), isTokenAutoRefreshEnabled: true })),
```

I am able to serve my application with both `yarn server` and `yarn dev:ssr` and I'm able to load the site with AppCheck seemingly working. However, when I try to run `yarn prerender`, I get the following error:

```
Unhandled Promise rejection: document is not defined ; Zone: ; Task: Promise.then ; Value: ReferenceError: document is not defined
```

issue に示されている AppCheck の初期化から始め、Angular SSR で `yarn prerender` を実行して失敗を再現します。prerendering 中にどの AppCheck 設定が `document` にアクセスしているかを追跡します。prerendering が `document is not defined` エラーなしで完了し、serving と開発用 SSR が引き続き動作すれば完了です。

索引モデルが issue の本文から書いたものです。