GH actions: purge expired artifacts

[juergba]

Description

Since recently the ESLint Check in GH actions is failing:
Error: HttpError: Failed to generate URL to download artifact
Error: Failed to generate URL to download artifact

GH does not delete expired artifacts.
https://api.github.com/repos/mochajs/mocha/actions/artifacts shows aprox. 20 artifacts, all of them expired.
When eslint-plus-action@v3.4.2 is trying to download an expired artifact, it fails. => see issue
It's unclear to me why we have artifact at all on that job.

{
  "total_count": 20,
  "artifacts": [
    {
      "id": 24440022,
      "node_id": "MDg6QXJ0aWZhY3QyNDQ0MDAyMg==",
      "name": "eslint-cache-key-lint-results-Tests-4498",
      "size_in_bytes": 5477,
      "url": "https://api.github.com/repos/mochajs/mocha/actions/artifacts/24440022",
      "archive_download_url": "https://api.github.com/repos/mochajs/mocha/actions/artifacts/24440022/zip",
      "expired": true,
      "created_at": "2020-11-03T05:04:09Z",
      "updated_at": "2020-11-03T05:04:17Z",
      "expires_at": "2021-02-01T04:53:25Z"
    },
....

Description

Add a scheduled action which deletes all expired artifacts.