Fix goroutine leak in game frontend

[govargo]

When I wrote the code with following space-agon's codebase, I found that my code has leaked goroutine in the container process.

I investigated and found that the following code in game frontend doesn't release the goroutine by e.g. break, return.

# current code
for {
  resp, err := stream.Recv()
  if err != nil {
    errs <- fmt.Errorf("error streaming assignment: %w", err)
    return
  }
  assignments <- resp.Assignment
}

This code leads to goroutine leak.
This PR fixes this problem.

I followed the example in open-match client example's code.
https://github.com/googleforgames/open-match/blob/main/examples/demo/components/clients/clients.go#L119-L126

[kemurayama]

LGTM