Accept multi-value input on slice flags

[vipally]

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 automatically downloading the latest version.

Describe the bug

Multi values on slice flags were acceted on ENV values input but flag values accept only single ones.

To reproduce

I have put the test code here

package main

import (
	"fmt"
	"os"

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

func main() {
	os.Args = []string{"multi_values",
		"--stringSclice", "parsed1,parsed2", "--stringSclice", "parsed3,parsed4",
		"--float64Sclice", "13.3,14.4", "--float64Sclice", "15.5,16.6",
		"--int64Sclice", "13,14", "--int64Sclice", "15,16",
		"--intSclice", "13,14", "--intSclice", "15,16",
	}
	os.Setenv("ENV_CLI_FLOAT64_SLICE", "23.3,24.4")
	app := cli.NewApp()
	app.Name = "multi_values"
	app.Flags = []cli.Flag{
		&cli.StringSliceFlag{Name: "stringSclice"},
		&cli.Float64SliceFlag{Name: "float64Sclice", EnvVars: []string{"ENV_CLI_FLOAT64_SLICE"}},
		&cli.Int64SliceFlag{Name: "int64Sclice"},
		&cli.IntSliceFlag{Name: "intSclice"},
	}
	app.Action = func(ctx *cli.Context) error {
		for i, v := range ctx.FlagNames() {
			fmt.Printf("%d-%s %#v\n", i, v, ctx.Value(v))
		}
		return ctx.Err()
	}

	_ = app.Run(os.Args)
}

// Incorrect Usage. invalid value "13.3,14.4" for flag -float64Sclice: strconv.ParseFloat: parsing "13.3,14.4": invalid syntax

Observed behavior

Incorrect Usage. invalid value "13.3,14.4" for flag -float64Sclice: strconv.ParseFloat: parsing "13.3,14.4": invalid syntax

Expected behavior

Output:

0-float64Sclice cli.Float64Slice{slice:[]float64{13.3, 14.4, 15.5, 16.6}, hasBeenSet:true}
1-int64Sclice cli.Int64Slice{slice:[]int64{13, 14, 15, 16}, hasBeenSet:true}
2-intSclice cli.IntSlice{slice:[]int{13, 14, 15, 16}, hasBeenSet:true}
3-stringSclice cli.StringSlice{slice:[]string{"parsed1", "parsed2", "parsed3", "parsed4"}, hasBeenSet:true}

Additional context

I have put the test code here

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Run go version and paste its output here

go version go1.15.6 windows/amd64

Run go env and paste its output here

set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\ally\AppData\Local\go-build
set GOENV=C:\Users\ally\AppData\Roaming\go\env
set GOEXE=.exe
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=C:\Users\ally\go\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=C:\Users\ally\go
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\ally\AppData\Local\Temp\go-build790788990=/tmp/go-build -gno-record-gcc-switches

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[stale]

Closing this as it has become stale.