Checks if an artifact with a given name, uploaded with actions/upload-artifact@v4, exists. If it does, the `exists` property is set on the `outputs` context.
`exists` will be set to `true` if it the artifact was downloaded, `false` it couldn't be downloaded. It could also be `null` if something in the composite workflow failed and it never couldn't got to the end, if the `download-artifact` call was skipped or something else strange happened.