fix the --no-pivot flag being ignored by ctr tasks start

Signed-off-by: Iceber Gu <wei.cai-nat@daocloud.io>

cmd/ctr/commands/tasks/tasks_unix.go

@@ -34,11 +34,11 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-func init() {
-	startCommand.Flags = append(startCommand.Flags, cli.BoolFlag{
+var platformStartFlags = []cli.Flag{
+	cli.BoolFlag{
 		Name:  "no-pivot",
 		Usage: "disable use of pivot-root (linux only)",
-	})
+	},
 }
 
 // HandleConsoleResize resizes the console