pkg/cri: replace some fmt.Sprintfs with strconv

Teeny-tiny optimizations:

    BenchmarkSprintf-10       37735996    32.31  ns/op  0 B/op  0 allocs/op
    BenchmarkItoa-10         591945836     2.031 ns/op  0 B/op  0 allocs/op
    BenchmarkFormatUint-10   593701444     2.014 ns/op  0 B/op  0 allocs/op

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/cri/streaming/remotecommand/exec.go

@@ -36,6 +36,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"strconv"
 	"time"
 
 	apierrors "k8s.io/apimachinery/pkg/api/errors"
@@ -76,7 +77,7 @@ func ServeExec(w http.ResponseWriter, req *http.Request, executor Executor, podN
 					Causes: []metav1.StatusCause{
 						{
 							Type:    remotecommandconsts.ExitCodeCauseType,
-							Message: fmt.Sprintf("%d", rc),
+							Message: strconv.Itoa(rc),
 						},
 					},
 				},