ci: Fix PR reporter for non-main branch targets

[rschristian]

Re: #4891 (comment). My guinea pig is still testing this, but seems like it'll work. My guinea pig has confirmed it'll work.

TLDR: PR events, when coming from those who have maintainer rights, include a field called pull_requests that looks like this:

github.event.workflow_run.pull_requests

"pull_requests": [
  {
    "base": {
      "ref": "master",
      "repo": {
        "id": 918443277,
        "name": "debug__workflow_run-fork",
        "url": "https://api.github.com/repos/rschristian/debug__workflow_run-fork"
      },
      "sha": "8a91303965a6e8f6ceba7a9a54c631a60b6d7ce5"
    },
    "head": {
      "ref": "debug/event-fields",
      "repo": {
        "id": 918443277,
        "name": "debug__workflow_run-fork",
        "url": "https://api.github.com/repos/rschristian/debug__workflow_run-fork"
      },
      "sha": "6cbbe6a0d854cf3c4817f22560b3c4434ac65457"
    },
    "id": 2634577244,
    "number": 10,
    "url": "https://api.github.com/repos/rschristian/debug__workflow_run-fork/pulls/10"
  }
]

Forked repo, on the other hand, gets this:

"pull_requests": []

...which is lame & there are no other references to the base SHA anywhere else in the event payload. Alternatively, one would think they could use GitHub's API to query for PRs containing a commit, or just query the commit & get the PRs that include it, but these too don't work for unknown reasons when the commit comes from a fork.

Instead, we're using the owner+branch name, querying for all PRs on the repo that match, getting the latest result & extracting the base SHA from it. This is weird but GitHub's data doesn't give us a choice.

[coveralls]

coverage: 99.535%. remained the same
when pulling bbadeba on ci/fix-pr-target
into 5dd6c09 on main.