avoid the use use of workflow.source_path

The above function returns a path to a cached source file, which
changes for every run, causing the resulting parameter to change and
the solve_network rule to re-run

rules/common.smk

@@ -43,7 +43,7 @@ def memory(w):
 def input_custom_extra_functionality(w):
     path = config["solving"]["options"].get("custom_extra_functionality", False)
     if path:
-        return workflow.source_path(path)
+        return os.path.join(os.path.dirname(workflow.snakefile), path)
     return []