my urfave/cli version is

v2.3.0

Checklist

• Are you running the latest v2 release? The list of releases is here.
• Did you check the manual for your release? The v2 manual is here
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Dependency Management

• My project is using go modules.
• My project is using vendoring.
• My project is automatically downloading the latest version.
• I am unsure of what my dependency management setup is.

Describe the bug

Flag names missing in cli.Context.FlagNames() if set via env. variables.

To reproduce

package main

import (
	"fmt"
	"github.com/urfave/cli/v2"
	"log"
	"os"
)

func main() {
	app := &cli.App{
		Flags: []cli.Flag{
			&cli.StringFlag{
				Name:    "testvar",
				EnvVars: []string{"TESTVAR"},
			},
		}, Action: func(c *cli.Context) error {
			fmt.Println("FlagNames()\t", c.FlagNames())
			fmt.Println("testvar\t\t", c.String("testvar"))
			return nil
		},
	}

	err := app.Run(os.Args)
	if err != nil {
		log.Fatal(err)
	}
}

Observed behavior

$ ./main --testvar testval
FlagNames()	 [testvar]
testvar		 testval
$ TESTVAR=testval ./main 
FlagNames()	 []
testvar		 testval

Expected behavior

I expect cli.Context.FlagNames() to return flag names irrespectively of which way they were set.

Run go version and paste its output here

$ go version
go version go1.16.2 linux/amd64