Reject a non-string execute command with a clear installer error

lutris/installer/commands.py

@@ -119,6 +119,11 @@ class CommandsMixin:
 
             exec_path = data.get("file", "")
             command = data.get("command", "")
+            if not isinstance(command, str):
+                raise ScriptingError(
+                    _("The command parameter of the execute command must be a string"),
+                    faulty_data=data,
+                )
             args_string = data.get("args", "")
             for arg in shlex.split(args_string):
                 args.append(self._substitute(arg))