Support using the `--preload` scheduler CLI flag alongside the `scheduler.preload` configuration options

This is not documented, but it seems that you cannot use the `--preload` scheduler CLI flag with the `distributed.scheduler.preload` configuration option.

The preload config is only read if _no_ `--preload` argument is used (https://github.com/dask/distributed/blob/2963bf12ec1fd27357360b01598f70fac2fe1750/distributed/scheduler.py#L3551-L3552).

You may have no control over the `--preload` option, or you want to set the preload option dynamically via environment variables. In this case it seems impossible to run any preload scripts on the scheduler.