CI: Switch to available latest images

`mirror.gcr.io/library/busybox:1.32` is unavailable.

```console
$ curl -s https://mirror.gcr.io//v2/library/busybox/tags/list | jq '.tags'
[
      "1.26.2",
      "1.27.2",
      "1.28",
      "1.29",
      "1.29.2",
      "1.29.3",
      "1.30",
      "1.30.1",
      "1.31",
      "1.31.0",
      "1.31.1",
      "1.32.0"
]
```

Signed-off-by: wllenyj <wllenyj@linux.alibaba.com>
(cherry picked from commit 6d961f9)
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

integration/client/client_unix_test.go

@@ -32,7 +32,7 @@ const (
 )
 
 var (
-	testImage    = "mirror.gcr.io/library/busybox:1.32"
+	testImage    = "mirror.gcr.io/library/busybox:1.32.0"
 	shortCommand = withProcessArgs("true")
 	longCommand  = withProcessArgs("/bin/sh", "-c", "while true; do sleep 1; done")
 )