SAFER does not work with sector studies

[trevorb1]

Checklist

• [X] I am using the current master branch
• [X] I am running on an up-to-date pypsa-usa environment. Update via conda env update -f envs/environment.yaml

The Issue

In sector studies, gas and coal thermal generators are converted to links to track fuel use between sectors. When adding SAFER constraints, only generators are queried, missing generators that have been converted to links.

Steps To Reproduce

No response

Expected Behavior

No response

Error Message

No response

Anything else?

https://github.com/PyPSA/pypsa-usa/blob/a05664935c3d0828b9dae46d64fcd4fca137d177/workflow/scripts/solve_network.py#L675-L753