Add actionlint to this repo's workflows to get better diagnostics/general lints

[tlater-famedly]

See actionlint. GitHub workflows have a good number of terse quirks, getting real error messages in CI would probably save us a bunch of time.

Also, we should point it out for local dev in the readme.

[tlater-famedly]

Sadly it's not available in the ubuntu repositories, so we can't currently use it in CI without violating the allowed software lists. I've put in a request to permit it.