GH actions issues

[bbolker]

it would be nice to have [ci skip] in commit message exit with an "OK" status from the whole job. If we use a zero error code for this step then the job keeps running. If we use a non-zero error code then the job status if 'failed' (ideally, I would like an indication that the job was skipped. This could probably be fine-tuned by learning more about GH actions and CI skip action and pissing around with the YAML appropriately.

For now I can live with skipped CI tests being reported as 'failed'.