osbuildexecutor: wait until worker-executor closes the connection

Otherwise the client will try to fetch the output archive before the
build output is archived on the worker-executor side.

internal/osbuildexecutor/runner-impl-aws-ec2.go

@@ -148,6 +148,11 @@ func handleBuild(inputArchive, host string) (*osbuild.Result, error) {
 		return nil, fmt.Errorf("Unable to decode response body into osbuild result: %w", err)
 	}
 
+	_, err = io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("Unable to wait for executor to close connection: %w", err)
+	}
+
 	return &osbuildResult, nil
 }