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

[ryan-roemer]

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

[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.