Support GitHub Deployments when displaying preview URLs

Preview links are not appearing for me because my CI/CD adds github metadata in the canonical location, but the github integration looks elsewhere.

**Describe the bug**

Currently, the Github integration looks at a commit status's contexts for a deploy url. The thing is, the canonical way to specify a deploy url is using environment deployments. So I think the integration should look at the environment deploy first, then fallback to the context target_url. Here's some GraphGL:

```gql
{
repository(name, owner) {
object(oid) {
... on Commit {
deployments(last: 1) {
nodes {
latestStatus {
environmentUrl # LOOK HERE FIRST
}
}
}
status {
contexts {
targetUrl # THEN LOOK HERE
}
}
}
}
}
}
```

**Expected behavior**

I expect the Github integrations that use the canonical location for deploy urls to work with NetlifyCMS.

**Screenshots**

Here's a screenshot of where Github displays such urls in the PR UI. See "View deployment":

**CMS configuration**

```js
{
publish_mode: 'editorial_workflow',

backend: {
name: 'github',
repo: 'MY/REPO',
branch: 'master',
squash_merges: true,
use_graphql: true,
open_authoring: false,
base_url: 'MYAUTHURL',
auth_endpoint: 'api/auth/request',
cms_label_prefix: 'cms/',
},
}
```

**Additional context**

In the meantime, do you know any workarounds? Thanks!

No files or tests are named. Start at the GitHub integration's GraphQL lookup for commit-status contexts and compare it with the deployment query shown in the issue. Done means environmentUrl is checked first and targetUrl remains the fallback, with coverage for both response paths.

Written by the indexing model from the issue text.