xpress: writesol instead of tofile

Author: fneum

linopy/solvers.py

@@ -890,7 +890,7 @@ def run_xpress(
 
     if solution_fn is not None:
         try:
-            m.tofile(path_to_string(solution_fn), filetype="sol")
+            m.writesol(path_to_string(solution_fn))
         except Exception as err:
             logger.info("Unable to save solution file. Raised error: %s", err)