bug: Cleanenv ignore environment variable when using pointer

Reproduceable steps:

config.yaml

foo: hello


- `main.go`
```go
package main

import (
    "fmt"

    "github.com/ilyakaznacheev/cleanenv"
)

type (
    Config struct {
        Foo *string `env-required:"true" yaml:"foo" env:"FOO"`
    }

)

func main() {
    cfg := new(Config)
    _ = cleanenv.ReadConfig("config.yaml", cfg)
    cleanenv.ReadEnv(cfg)

    fmt.Println(*cfg.Foo)
}

export FOO=bar

it will print "hello" instead of bar

is this limitation of reflection??

ilyakaznacheev / cleanenv

bug: Cleanenv ignore environment variable when using pointer #147