fix: 'Save Chart' modal's dashboard dropdown isn't sticky (#11338)

It appears that the logic that makes the dashboard selection sticky
in the 'Save Chart' modal (explore) was broken at some time.

While working on a dashboard, it's nice if this modal can remember the
dashboard selection.

superset/views/core.py

@@ -873,6 +873,7 @@ class Superset(BaseSupersetView):  # pylint: disable=too-many-public-methods
             "form_data": slc.form_data,
             "slice": slc.data,
             "dashboard_url": dash.url if dash else None,
+            "dashboard_id": dash.id if dash else None,
         }
 
         if dash and request.args.get("goto_dash") == "true":