Add a check for jobs running locally or in the cloud

[ejulio]

Currently, we may have settings for spiders running locally and other settings for spiders running in Scrapy Cloud (Dash).
Usually, I add a check like if 'SHUB_JOBKEY' not in os.environ:.
However it may not be the best one and , if for some reason this env is deprecated, I need to update my checks.
If would be nice to have this kind of check, not sure if in this library or somewhere else.