nixos/buildkite-agent: tweak TimeoutStopSec and KillMode

This improves behaviour when the service is being stopped.

nixos/modules/services/continuous-integration/buildkite-agent.nix

@@ -228,6 +228,9 @@ in
             RestartSec = 5;
             Restart = "on-failure";
             TimeoutSec = 10;
+            # set a long timeout to give buildkite-agent a chance to finish current builds
+            TimeoutStopSec = "2 min";
+            KillMode = "mixed";
           };
       };