solve_network: make sure infeasibilities are printed properly

Without this formatting, there is an error adding a string to a list.

scripts/solve_network.py

@@ -926,7 +926,7 @@ def solve_network(n, config, solving, opts="", **kwargs):
         )
     if "infeasible" in condition:
         labels = n.model.compute_infeasibilities()
-        logger.info("Labels:\n" + labels)
+        logger.info(f"Labels:\n{labels}")
         n.model.print_infeasibilities()
         raise RuntimeError("Solving status 'infeasible'")