That __cloud:UserId is the GUID for the User in the MongoDB. Assuming you have default MongoDB under the hood on your server, you can use a MongoDB Input to pull the Users collection and match the ID to the user. When setting up the MongoDB for this, you will need the MongoDB admin password from the System Settings, otherwise you can ask IT to dump that collection to a DW table each week and hit it from there, but you then have your user table sitting somewhere else that needs to be protected.
The reason that the GetEnvironmentVariable did not work, is the user that it will be running under is a default one on the server unless you specifically have the workflow set up to run under user credentials (I don't recommend unless you need it, because it requires a lot more management). There is a difference between windows user and Alteryx user.
