Make runners configurable in ci

our ci doesnt allow you to change much of the ci itself in a pr, but we do have .github/config.yaml which is intended to allow some types of change - atm you can change the docker container used by ci eg

what would be really helpful is to be able to set the runners - atm the immediate use case is for testing different macos runners - but its come up plenty in other contexts

implementation shouldnt be too hard - but defo requires off-site testing