fix: use Key instead of OwnKey in rawEnvVars to properly reference existing defaults in complex structs

[AndrewChubatiuk]

currently it's impossible to expand variable from other struct field, if configuration struct is not plain
in a given below example Config.Component.Endpoint.Value can be only obtained from env variable, but not from default value of Config.Component.Host

type Config struct {
  Name string `env:"NAME" envDefault:"some-name"`
  Component struct {
    Host string `env:"HOST" envDefault:"localhost"`
    Endpoint struct {
      Value string `env:"VALUE,expand" envDefault:"${COMPONENT_HOST}"`
    }
  } `envPrefix:"COMPONENT_"`
}