Convert the root .travis.yml file into a GitHub Actions workflow

[kyluca]

Implements #214.

Hopefully this is in line with what you were looking for @jacebrowning?

• It should be roughly the same as the Travis script except for the second clone operation which I moved out into a dedicated step of its own using a native GHA action instead of a raw git clone.
• I've also deferred the deletion of the old .travis.yml file to leave an overlap period where you can verify the new workflow is working as expected before disabling Travis.
• I'm not sure if I can effectively test the demo repo integration from my fork, so it might be best to try the workflow from the original repo as well, hoping the default built-in tokens have the right scopes and permissions etc.

Let me know if there's anything I've missed :)

[jacebrowning]

Where does this value come from?

[kyluca]

This is the unique user ID of the official GHA bot account, taken from the link I put as a comment on L48: https://github.com/actions/checkout?tab=readme-ov-file#push-a-commit-using-the-built-in-token

That doc in turn references the user ID directly from the GH API https://api.github.com/users/github-actions%5Bbot%5D.

Here's another discussion which may explain better: https://github.com/orgs/community/discussions/26560#discussioncomment-3252340

[jacebrowning]

I'm trying to figure out how to make the token work here: #216