The Workflow ID refers to a specific workflow in your Retool application and stays constant (as you noted) and it can be found in your workflow settings or under workflowContext.workflowId.
Conversely, the Workflow Run ID is generated each time a workflow is executed, allowing you to track specific runs of a workflow. This can be viewed in the Run History section of a workflow in the Workflow IDE. Click on "Run history" in the bottom left and open up the "Logs" tag to find the Workflow Run Ids.
With that said, I don't believe there's currently a way to get {{Workflow Run Id}}, but I just created a feature request for it