fix(gs/mugwump): Add missing scheme to grafana datasource

Change-Id: I954fcca422f2e1325c2455cb1c4d77d53673901f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2061
Reviewed-by: glittershark <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/glittershark/system/system/machines/mugwump.nix

@@ -88,7 +88,7 @@ with lib;
       datasources = [{
         name = "Prometheus";
         type = "prometheus";
-        url = "localhost:9090";
+        url = "http://localhost:9090";
       }];
     };
   };