fix: make exec-id flag required in exec command

Signed-off-by: jonyhy <yun.hao@daocloud.io>

cmd/ctr/commands/tasks/exec.go

@@ -31,7 +31,6 @@ import (
 	"github.com/urfave/cli"
 )
 
-//TODO:(jessvalarezo) exec-id is optional here, update to required arg
 var execCommand = cli.Command{
 	Name:           "exec",
 	Usage:          "execute additional processes in an existing container",
@@ -52,6 +51,7 @@ var execCommand = cli.Command{
 		},
 		cli.StringFlag{
 			Name:     "exec-id",
+			Required: true,
 			Usage:    "exec specific id for the process",
 		},
 		cli.StringFlag{