shim-v2: Fix gosimple issue on utils_test.go

For some reason our static check started to get opinionated about code
that's been there for ages.

One of the suggestions is to improve:

```
INFO: Running golangci-lint on /home/fidencio/go/src/github.com/kata-containers/kata-containers/src/runtime/containerd-shim-v2
utils_test.go:76:36: S1039: unnecessary use of fmt.Sprintf (gosimple)
	testDir, err = ioutil.TempDir("", fmt.Sprintf("shimV2-"))
```

And that's what this PR is about.

Fixes: #2204

Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>

src/runtime/containerd-shim-v2/utils_test.go

@@ -73,7 +73,7 @@ func init() {
 	var err error
 
 	fmt.Printf("INFO: creating test directory\n")
-	testDir, err = ioutil.TempDir("", fmt.Sprintf("shimV2-"))
+	testDir, err = ioutil.TempDir("", "shimV2-")
 	if err != nil {
 		panic(fmt.Sprintf("ERROR: failed to create test directory: %v", err))
 	}