Improve testability using Act

[fenech]

Is your feature request related to a problem? Please describe.

We have found it difficult to test these workflows using https://github.com/nektos/act, since it relies on getting a real PR, rather than using the information in the PR event body.

Describe the solution you'd like

We would like to be able to send a minimal pull request payload like:

{
  "pull_request": {
    "title": "feat: do stuff",
    "labels": []
  }
}

and verify that our workflow is correctly configured before adding it to our repo.

Describe alternatives you've considered

The alternative is to make a real PR in GitHub and send a payload referencing it:

{
  "pull_request": {
    "number": 123,
    "base": {
      "user": {
        "login": "fenech"
      }
    },
    "repo": {
      "name": "my-repo"
    }
  }
}

The motivation for fetching the PR is discussed in #218, but it seems that the solution is to change the concurrency settings of the workflow. Doing that would improve the testability of this Action.