v2 bug: default value changes with parsed values on slice flags

[vipally]

my urfave/cli version is

v2

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
• Did you perform a search about this problem? Here's the Github guide about searching.

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

default value changes with parsed values on slice flags.

To reproduce

I have put the test code here

package main

import (
	"fmt"

	"github.com/urfave/cli/v2"
)

func main() {
	app := &cli.App{
		Name: "cli_bug",
		Flags: []cli.Flag{
			&cli.BoolFlag{
				Name:    "bool",
				Value:   false,
				Aliases: []string{"b"},
			},
			&cli.StringFlag{
				Name:    "string",
				Aliases: []string{"s"},
				Value:   "default",
			},
			&cli.IntFlag{
				Name:    "int",
				Aliases: []string{"i"},
				Value:   1,
			},
			&cli.StringSliceFlag{
				Name:    "stringSlice",
				Aliases: []string{"ss"},
				Value:   cli.NewStringSlice("default1", "default2"),
			},
			&cli.Float64SliceFlag{
				Name:    "float64Slice",
				Aliases: []string{"f64s"},
				Value:   cli.NewFloat64Slice(1.1, 2.2),
			},
			&cli.Int64SliceFlag{
				Name:    "int64Slice",
				Aliases: []string{"i64s"},
				Value:   cli.NewInt64Slice(1, 2),
			},
			&cli.IntSliceFlag{
				Name:    "intSlice",
				Aliases: []string{"is"},
				Value:   cli.NewIntSlice(1, 2),
			},
		},
		Action: func(ctx *cli.Context) error {
			fmt.Println("FlagNames", ctx.FlagNames())
			for i, v := range ctx.FlagNames() {
				fmt.Printf("%d %s %+v\n", i, v, ctx.Value(v))
			}
			err := ctx.Err()
			fmt.Println("error", err)
			if err == nil {
				cli.ShowAppHelp(ctx)
			}
			return nil
		},
	}
	args := []string{
		"lab23",
		"-b", // "false",
		"--string", "setstring1",
		"--int", "13",
		"--stringSlice", "stringSlice",
		"--float64Slice", "13.3",
		"--int64Slice", "13",
		"--intSlice", "13",
	}
	fmt.Printf("args: %+v\n\n", args)
	if err := app.Run(args); err != nil {
		panic(err)
	}

	/*
		GLOBAL OPTIONS:
		   --bool, -b                          (default: false)
		   --string value, -s value            (default: "default")
		   --int value, -i value               (default: 1)
		   --stringSlice value, --ss value     (default: "stringSlice") // bug expect: (default: "default1", "default2")
		   --float64Slice value, --f64s value  (default: 13.3)			// bug expect: (default: 1.1, 2.2)
		   --int64Slice value, --i64s value    (default: 13)			// bug expect: (default: 1, 2)
		   --intSlice value, --is value        (default: 13)			// bug expect: (default: 1, 2)
		   --help, -h                          show help (default: false)
	*/
}

Observed behavior

The default value of slices changes from the parsed value:
--stringSlice value, --ss value (default: "stringSlice") // bug expect: (default: "default1", "default2")
--float64Slice value, --f64s value (default: 13.3) // bug expect: (default: 1.1, 2.2)
--int64Slice value, --i64s value (default: 13) // bug expect: (default: 1, 2)
--intSlice value, --is value (default: 13) // bug expect: (default: 1, 2)

Expected behavior

Expected default values:
--stringSlice value, --ss value (default: "default1", "default2")
--float64Slice value, --f64s value (default: 1.1, 2.2)
--int64Slice value, --i64s value (default: 1, 2)
--intSlice value, --is value (default: 1, 2)

Additional context

Add any other context about the problem here.

If the issue relates to a specific open source Github repo, please link that repo here.

If you can reproduce this issue with a public CI system, please link a failing build here.

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Run go version and paste its output here

go version go1.15.3 windows/amd64

Run go env and paste its output here

set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\Admin\AppData\Local\go-build
set GOENV=C:\Users\Admin\AppData\Roaming\go\env
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=D:\dev\gocode\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=D:\dev\gocode
set GOPRIVATE=
set GOPROXY=https://goproxy.cn
set GOROOT=c:\go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLDIR=c:\go\pkg\tool\windows_amd64
set GCCGO=gccgo
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=1
set GOMOD=
set CGO_CFLAGS=-g -O2
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-g -O2
set CGO_FFLAGS=-g -O2
set CGO_LDFLAGS=-g -O2
set PKG_CONFIG=pkg-config
set GOGCCFLAGS=-m64 -mthreads -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=C:\Users\Admin\AppData\Local\Temp\go-build849182602=/tmp/go-build -gno-record-gcc-switches

[vipally]

maybe related to #1206