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_"`
}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (1ff55e6) to head (fe53c87).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #412   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          532       532           
=========================================
  Hits           532       532           

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