Add GHA workflow validation

[schlessera]

Use the Action Validator to do some initial smoke testing of the GitHub Actions workflows.

[swissspidy]

Shouldn't I be seeing this job now in the successful checks list? 🤔

Given that all the commands just use the reusable workflows from this repo, there doesn't seem to be too much value in running this for all of them, but I guess it can't hurt. Seems more important to ensure the reusable workflows match the schema.

[schlessera]

This will not show up just yet as the action pulls the reusable workflows from the main branch. Once we merge this, the change will be part of the main branch and appear in all subsequent runs.

I thought it might make sense to include for all repos to make sure PRs don't mess up the testing.yml file that is meant to pull in the reusable one. Not sure how much value this provides, but the more checks the merrier, I guess?

[swissspidy]

@schlessera So that didn't quite work 🤔

https://github.com/wp-cli/.github/actions/runs/6666231157/job/18117347330

Weird that it says it's not installed after just installing it 🤷

---

Aside:

I already had the tool installed via npm, but I just realized there it doesn't have a --verbose option 🤔