Improve the daily.yml workflow

[AlexWaygood]

1. Make it easier to run the workflow from a typeshed fork using the workflow_dispatch trigger. This is something I do sporadically if a mypy release is coming up, so we don't get blindsided by loads of very disruptive stubtest changes when the release is cut. Making this change means I don't have to manually edit the workflow to run it from my private fork, which is a pain :)
2. Auto-add the "help wanted" label to issues created by the bot when the daily test fails. @kkirsche brought up in Evaluate ways to determine accuracy and reduce incomplete type stubs #8481 (comment) that it's sometimes pretty difficult for contributors to know which typeshed issues we'd welcome help on, and which issues aren't really actionable, or are pretty out of date. I think that's a very reasonably criticism, and I've been trying to improve on that by using the "help wanted" label more often, to indicate issues where a maintainer has confirmed that a PR to fix the issue would be welcome. Pretty much any issue auto-created by the bot due to a stubtest failure can be considered "auto-triaged", so let's automatically add the label to those issues.

(github.event_name is documented here. Using the GitHub API to create issues with specific labels is documented here.)