ci(pr-review-companion): identify PR number from head repository

[caugner]

Description

Update the pr-review-companion workflow, identifying the PR number from the head repository instead of the base repository.

Motivation

The current approach fails to identify the PR number for fork PRs, because the commits API only returns commits in the given repo, excluding forks.

Additional details

Noticed in #1221 that the PR number was empty. And here's is the explanation:

$ gh api repos/mdn/fred/commits/56a905455f8be1a8d8194089c2cbaf2a0debd455/pulls
[]

$ gh api repos/mayank99/fred/commits/56a905455f8be1a8d8194089c2cbaf2a0debd455/pulls
[
  {
    "url": "https://api.github.com/repos/mdn/fred/pulls/1221",
    // …
    "number": 1221,
    // …
    "base": {
      // …
      "repo": {
        // …
        "full_name": "mdn/fred",
        // …
      }
    },
    // ...
  }
]

Related issues and pull requests

[github-actions]

449c88d was deployed to: https://fred-pr1258.review.mdn.allizom.net/