Use Prettier for our YAML files

The formatting in the GitHub-workflow files is not consistent; some files use two spaces for indentation, some use four.

We should have a `.prettierrc` and, if we don't like whatever the defaults are, [specify some overrides](https://prettier.io/docs/en/configuration.html#configuration-overrides) for certain file extensions. We should also add Prettier on a pre-commit hook or something.