Bug: Infer changeId in a non-caught-up branch PR

When we have an unmergeable / out-of-sync branch in an open PR, formideploy doesn't infer `changeId` correctly and skips the staging deploy. See, e.g. https://github.com/FormidableLabs/spectacle/runs/6582133835

```js
[config] Full configuration:
{
"lander": {
"name": "spectacle"
},
"github": {
"org": "FormidableLabs",
"repo": "spectacle"
},
"build": {
"dir": "build",
"path": "build/open-source/spectacle",
"jobId": "2378867420",
"jobUrl": "https://github.com/FormidableLabs/spectacle/actions/runs/2378867420",
"changeId": null,
"isPullRequest": false
},
"site": {
"basePath": "open-source/spectacle",
"redirects": {},
"excludes": []
},
"staging": {
"domain": "formidable-com-spectacle-staging-null.surge.sh"
},
"production": {
"domain": "formidable.com",
"bucket": "formidable.com",
"region": "us-east-1"
}
}

[formideploy] Starting {"action":"staging"} with options
{
"port": 5000,
"limit": 10,
"start": "2022-05-24T21:44:09.045Z",
"dryrun": false,
"production": false,
"staging": true
}

[deploy:staging] Skipping deployment (CI): CI did not provide a build.changeId value (e.g., merge or master build)
```

Task:

- [ ] Infer `changeId` in GH actions for unmergeable PR branches.