module-eval: give the Grafana stand-in a mainProgram, so upstream's getExe stops warning

nix/module-eval/lib.nix

@@ -301,9 +301,17 @@ let
       );
     in
     builtins.foldl' (acc: v: builtins.seq v acc) true vals;
+
+  # A stand-in for the Grafana package in the cases that do not run it. Upstream's
+  # Grafana module takes the executable's name from `meta.mainProgram`, and
+  # warns when a package has none.
+  grafanaStub = pkgs.emptyDirectory // {
+    meta.mainProgram = "grafana";
+  };
 in
 {
   inherit hive hiveWith;
+  inherit grafanaStub;
   inherit hiveWithController;
   inherit agent;
   inherit agentWith;