That’s intentionally not possible. (There are cases where the doc has to be run through the sync function by internal maintenance processes, not by the user who modified the document; in that case a value like “currentUserID” would end up being null or otherwise non-useful.)
What you do instead is have the client app put the user’s ID into a document property, something like owner. Then the sync function should call requireUser(doc.owner) to validate that the user changing the document is the document’s owner. (This works even during maintenance, because in that mode requireUser() is a no-op.)
Now you have the user’s ID in the doc.owner property and can use it as in your snippet above, to derive a channel name.