cmd/*: refactor get flag value (#24761)

cmd/devp2p/enrcmd.go

@@ -34,18 +34,20 @@ import (
 	"gopkg.in/urfave/cli.v1"
 )
 
+var fileFlag = cli.StringFlag{Name: "file"}
+
 var enrdumpCommand = cli.Command{
 	Name:   "enrdump",
 	Usage:  "Pretty-prints node records",
 	Action: enrdump,
 	Flags: []cli.Flag{
-		cli.StringFlag{Name: "file"},
+		fileFlag,
 	},
 }
 
 func enrdump(ctx *cli.Context) error {
 	var source string
-	if file := ctx.String("file"); file != "" {
+	if file := ctx.String(fileFlag.Name); file != "" {
 		if ctx.NArg() != 0 {
 			return fmt.Errorf("can't dump record from command-line argument in -file mode")
 		}