feat: make all cli flags available as env variables (opt-in)

[mfridman]

This might not be necessary since it's just a few helper functions most users can stub out in their code, e.g.,

func getFlagOrEnv(s *cli.State, name string, envPrefix string) (string, error) {
	envName := strings.ToUpper(name)
	val := cmp.Or(
		cli.GetFlag[string](s, name),
		os.Getenv(envPrefix+envName),
	)
	if val == "" {
		return "", fmt.Errorf("must provide --%s flag or set %s environment variable", name, envName)
	}
	return val, nil
}

We could optionally bake this logic into the cli library or add a cli.GetFlagOrEnv helper function.