thanos: add meta.mainProgram

pkgs/servers/monitoring/thanos/default.nix

···
     homepage = "https://github.com/thanos-io/thanos";
     changelog = "https://github.com/thanos-io/thanos/releases/tag/v${version}";
     license = licenses.asl20;
+    mainProgram = "thanos";
     maintainers = with maintainers; [ basvandijk anthonyroussel ];
   };
 }