ci(pr-review-companion): identify PR via GitHub API

[caugner]

Description

Refines the pr-review-companion workflow:

1. Identifies the PR number via the GitHub API, instead of using the NR file in the build artifact.
2. Reports the review deployment status on fork PRs as well, using the head_sha from the workflow_run payload.

Motivation

1. Improve the security of the workflow.
2. Make review deployment status visible on fork PRs.

Additional details

Related issues and pull requests

Same approach as in Fred:

• ci(pr-review-companion): support fork PRs fred#1235
• ci(pr-review-companion): identify PR number from head repository fred#1258

[caugner]

Test failure is unrelated:

 Fetching list of changed files for PR#43072 from Github API
  Invoking listFiles(pull_number: 43072, per_page: 100)
Error: This endpoint is temporarily being throttled. Please try again later. For more on scraping GitHub and how it may affect your rights, please review our Terms of Service (https://docs.github.com/en/site-policy/github-terms/github-terms-of-service)

[LeoMcA]

Looks good.

I wonder if it's worth moving this identify PR step into an action/reusable workflow: as we're using it across a few repos now.