Add the possibility to give environment variables to dask client in 'dask.get_or_create_dask_client'

[remi-braun]

See https://jacobtomlinson.dev/posts/2022/how-to-set-environment-variables-on-your-dask-workers/

Currently we have to do:

def set_env():
    os.environ["ENV_VAR"] = "1"

with dask.get_or_create_dask_client(processes=False) as client:
    client.run(set_env)
    function(*args, **kwargs)

Goal is to do:

with dask.get_or_create_dask_client(env_vars={"ENV_VAR": "1"}) as client:
    client.run(set_env)
    function(*args, **kwargs)