[benchmark] Fix a flaky stack-use-after-return bug (#37079)

Closes #37079

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37079 from ctiller:asan-flake ea6cda4610
PiperOrigin-RevId: 647444711

test/core/transport/call_spine_benchmarks.h

@@ -220,12 +220,13 @@ void BM_ClientToServerStreaming(benchmark::State& state) {
     handler_done.WaitForNotification();
     initiator_done.WaitForNotification();
   }
-  call.initiator.SpawnInfallible("done", [&]() {
-    call.initiator.Cancel();
-    return Empty{};
-  });
-  call.handler.SpawnInfallible("done", [&]() {
-    call.handler.PushServerTrailingMetadata(
+  call.initiator.SpawnInfallible("done",
+                                 [initiator = call.initiator]() mutable {
+                                   initiator.Cancel();
+                                   return Empty{};
+                                 });
+  call.handler.SpawnInfallible("done", [handler = call.handler]() mutable {
+    handler.PushServerTrailingMetadata(
         CancelledServerMetadataFromStatus(absl::CancelledError()));
     return Empty{};
   });