windows/svc: fix printf(var) mistake detected by latest printf checker

For golang/go#69267.

Change-Id: Ie240b5c826bb96c0e2021a7e99a3c0f973f0a0e1
Reviewed-on: https://go-review.googlesource.com/c/sys/+/610940
Reviewed-by: Tim King <taking@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

windows/svc/svc_test.go

@@ -178,7 +178,7 @@ func TestIsWindowsServiceWhenParentExits(t *testing.T) {
 		child.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=child")
 		err := child.Start()
 		if err != nil {
-			fmt.Fprintf(os.Stderr, fmt.Sprintf("child start failed: %v", err))
+			fmt.Fprintf(os.Stderr, "child start failed: %v", err)
 			os.Exit(1)
 		}
 		os.Exit(0)