Specs calculation occurs outside of StreamingWorkunit plugin scope

[stuhood]

On 2.13.x, specs calculation is seemingly occurring outside of the StreamingWorkunit plugin's scope. All of the workunits are still handled, but the plugin doesn't receive its first call until after the specs have been calculated and the rest of the run is starting.

[stuhood]

Hm, apparently it has always been this way, because the StreamingWorkunitHandler constructor actually takes the constructed specs. To fix this, I think that we'll need to fix #13757, which will make the specs a calculated value to be used in get_expanded_specs, rather than a pre-computed value.