Deploy Preview Links Not Working With AWS Amplify Preview Builds

**Describe the bug**
Preview link to the the AWS created URL sent to the github check is not displayed in the Editor.

**To Reproduce**
1. Setup NetlifyCMS to work with AWS Amplify.
2. Enable AWS Amplify Preview builds, that trigger when a PR is created for your branch (When you make a change in the NetlifyCMS editor)
3. Can see the new provided URL in the Github check.
4. View Preview button does not display on the Editor and so cannot preview the changes before publishing the changes.

**Expected behavior**
I would expect for the preview button to display in the editor and to be able to click it to be taken to the URL of the unpublished changes.

**Screenshots**

**Applicable Versions:**

- netlify-cms-app 2.14.19
- netlify-cms-core 2.37.1
- Git provider: GitHub
- OS: Windows 10
- Chrome

**CMS configuration**
```
"backend": {
"name": "github",
"repo": "R/P",
"branch": "master",
"site_domain": "https://X1.com",
"base_url": "https://X2.com",
"auth_endpoint": "/prod/auth"
},
"load_config_file": false,
"site_url": "https://X3.com",
"display_url": "https://X3.com",
"media_folder": "/public/img",
"public_folder": "img",
"publish_mode": "editorial_workflow",
```