ERROR running force:source:pull: Entity of type 'Workflow' named 'Workflow' cannot be found

To avoid the error on running sfdx force:source:pull run the following query through the tooling API and delete any records it returns (easily done in Dev Console):

SELECT Id, MemberType, MemberName FROM SourceMember WHERE MemberType = 'Workflow'

If you have any workflow metadata which you need to retrieve then use sfdx force:source:retrieve to pull these from the scratch org.

It's a known bug that hasn't been fixed, you can follow progress over here: https://success.salesforce.com/issues_view?id=a1p3A000001SitrQAC

EDIT: Fixed as of December 2019