fix: Load panics when trying to load a non primitive type field with a string inner type

[riftsin]

Hello everyone,

I recently stumbled across a bug when using this library, the following code panics:

	type myStringType string
	type myTestConfig struct {
		Name myStringType
	}
	var result myTestConfig
	key := "MYTEST_NAME"
	expected := "bob"
	os.Setenv(key, expected)
	New(&Config{
		ENVPrefix: "MYTEST",
	}).Load(&result) // panic

This MR fixes this issue.