Race condition in WorkflowJob output callback when using MultithreadedJobExecutor

Continuation of #2003

## Expected Behavior
Scatter outputs should be collected only once for each scatter job.

## Actual Behavior
The patch provided by @GlassOfWhiskey in PR #2051 has done well to ensure the expected behavior, but as they note, it doesn't address the root cause. Under certain conditions, `ReceiveScatterOutput.receive_scatter_output()` may still be called twice within a narrow window of time for the same scatter job output.

The root cause lies within `WorkflowJob` and the conditions it uses to determine if `.do_output_callback()` should be called. This method is intended to be called from **either** `WorkflowJob.receive_output()` **OR** `WorkflowJob.job()`, but during the race condition this method is called from both.

`WorkflowJob.job()` runs in one thread while `WorkflowJob.receive_output()` is the callback bound to a work unit being executed by one of the `TaskQueue` workers, i.e. it executes in a separate scatter job thread. The race condition is that `.receive_output()` thread calls `do_output_callback()` (which later sets `self.did_callback = True`) and while in the body of that method, the `.job()` thread queries the value of `did_callback` which is still False, so it also calls `do_output_callback()`.

The relevant shared state is:
- `WorkflowJob.did_callback`
- `WorkflowJob.steps` where completed==True

I wanted to be sure that both methods were executing the same callback, which can be a little tricky with all of the nested functools partials that obscure the object to which each level is bound. If you unwind the callback chain you'll see they are the same in both branches:
```Python
WorkflowJob.receive_output() # [workflow scatterletters_#]
WorkflowStep.receive_output() # [simple-simple-scatter.cwl#scatterletters]
ReceiveScatterOutput.receive_scatter_output()
WorkflowJob.receive_output() # [workflow ]
MultithreadedJobExecutor.output_callback()
```
```Python
WorkflowJob.job() # [workflow scatterletters_#]
WorkflowStep.receive_output() # [simple-simple-scatter.cwl#scatterletters]
ReceiveScatterOutput.receive_scatter_output()
WorkflowJob.receive_output() # [workflow ]
MultithreadedJobExecutor.output_callback()
```

## Workflow Code
See #2003