Possible incorrect commodity prices when assets have more than one output commodity with `pricing_strategy` == `scarcity`

Marking this as a question, because it isn't a *confirmed* bug. @ahawkes has suggested that it could be a problem though, so I've opened this issue to track it. We should investigate.

From Adam in https://github.com/EnergySystemsModellingLab/MUSE_2.0/issues/663#issuecomment-3019439462:

> While doing all this I did have one further concern. I wonder if the non-scarcity pricing will work ok when there is more than one output SED/SVD commodity? Don't have time to check that right now, so could we raise it as a possible issue - to be checked soon?