Run the grafana workflow every week

This isn't running on main after dependabot automerges it at the moment, so
we're doing a manual check each week until we can figure it out.

See #61

.github/workflows/grafana.yml

@@ -9,6 +9,9 @@ env:
 
 on:
   workflow_dispatch:
+  schedule:
+    # Run every Wednesday at 8am
+    - cron: "0 8 * * 3"
   push:
     paths:
       - 'grafana/**'