Values are not loaded from environment with SetDefaultsForZeroValuesOnly

The change introduced in 2be9960e75cc5f053dcb89c64e43de19765fd154 causes values to no longer load from the environment in the presence of `SetDefaultsForZeroValuesOnly: true` and a non-zero value for the corresponding field.

The following is a quick example to recreate the problem:
``` go
type Config struct {
Username string `env:"USERNAME" envDefault:"admin"`
}
cfg := Config{Username: "root"}
err := env.ParseWithOptions(&cfg, env.Options{
Environment: map[string]string{"USERNAME": "user1"},
SetDefaultsForZeroValuesOnly: true,
})
if err != nil {
fmt.Println(err)
return
}
fmt.Printf("%+v", cfg) // prints `{Username:root}`, but should print `{Username:user1}`
```

The relevant logic is here:
https://github.com/caarlos0/env/blob/56a09d295d9321b1f3b537fd23df1527011cd83d/env.go#L510-L512

It does not check to ensure that `value` was loaded from `envDefault` rather than from the actual environment.

Thank you for the great library!

Start with the ParseWithOptions path in env.go around lines 510-512 and reproduce the provided example using SetDefaultsForZeroValuesOnly. Trace whether the value came from envDefault or the environment; done means the environment value is loaded despite the pre-existing field value, with the existing Go tests passing.

Written by the indexing model from the issue text.