Adds actionlint to lint GitHub Actions workflows

[JayShortway]

As the title says. Needed, as GitHub doesn't surface errors, and I find myself going back and forth to https://rhysd.github.io/actionlint/ to fix. So I figured it's better to catch these automatically before they're committed.

[tonidero]

Oh interesting. I guess it will still let you push, but will just give an error right? In any case, this seems useful 👍

[JayShortway]

@tonidero Yeah exactly. I didn't add it to the required checks, because I made it to only run when the GitHub Actions workflows actually change. If we want it to always run, we can add it to the required checks.