Automatic refreshing of GHC versions

Currently the workers just make the host .ghcup directory available in the containers, meaning that it's up to the worker host administrator to install new GHC versions in ghcup whenever they get released. (Furthermore, said administrator should remember to install ubuntu-compatible ones, if the host OS is not ubuntu.)

It would be nice to have some sort of automatic installation of new GHCs, including cleanup of older minor versions. Perhaps this should also mean that the worker has its own separate .ghcup directory?