fix: apply envPrefix to all BindEnv variable names

[justintoman]

I am new to Go, so apologies if I did things in a silly way.

I noticed the env prefix does not get applied to the ENV variable names in the case of more than one argument being passed to BindEnv. I tried to fix this and add a test case to verify.

os.Setenv("APP_FOO", "with prefix")
os.Setenv("FOO", "without prefix")

viper.SetEnvPrefix("APP")
viper.AutomaticEnv()

viper.BindEnv("a.b", "FOO")


viper.Get("a.b") // before: "without prefix", after: "with prefix"

type Nested struct {
  b  string `mapstructure:"b"`
}

type Root struct {
  a Nested `mapstructure:"a"`
}

var a Root
viper.Unmarshal(a)
a.b // before: "without prefix", after: "with prefix"

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