Additional updates for detecting a distributed scheduler

[jrbourbeau]

Following dask/distributed#7482 we're no longer using the config system to say that a distributed scheduler is available. Almost all of the relevant changes needed here were included in #9808. This PR includes two similar updates (only noticed these because I saw related tests beginning to fail on main).

cc @fjetter for visibility

[jrbourbeau]

For reference, these are the test failures this PR is fixing

FAILED dask/tests/test_distributed.py::test_zarr_in_memory_distributed_err - Failed: DID NOT RAISE <class 'RuntimeError'>
FAILED dask/tests/test_distributed.py::test_to_hdf_distributed - UserWarning: Running on a single-machine scheduler when a distributed client is active might lead to unexpected results.

I'll plan to merge this PR after CI finishes so it can be included in the release later today

[jrbourbeau]

Going to go ahead and merge this so we can get the release out (all the macOS machines are currently occupied). This issue isn't mac-specific and was impacting other OS builds too. Given linux and windows builds have already passed, I have confidence this issue is also solved on macOS.

EDIT: I also ran those two test locally on my mac and they both passed