Clear Template Variables from scheduler options before reload (#5996)

Clear -S and -D from options to force reload

* Closes #5968

cylc/flow/scheduler.py

@@ -1267,6 +1267,10 @@ def _configure_contact(self) -> None:
     def load_flow_file(self, is_reload=False):
         """Load, and log the workflow definition."""
         # Local workflow environment set therein.
+        # Allow -S and -D to take effect in Cylc VR.
+        # https://github.com/cylc/cylc-flow/issues/5968
+        self.options.rose_template_vars = []
+        self.options.defines = []
         return WorkflowConfig(
             self.workflow,
             self.flow_file,