Fix double unwrapped WDL implementation of basename operator

Author: illusional

janis_core/operators/standard.py

@@ -142,7 +142,7 @@ def to_python(self, unwrap_operator, *args):
 
     def to_wdl(self, unwrap_operator, *args):
         arg = unwrap_operator(args[0])
-        return f"basename({unwrap_operator(arg)})"
+        return f"basename({arg})"
 
     def to_cwl(self, unwrap_operator, *args):
         arg = unwrap_operator(