Follow XDG paths instead of $HOME

[igoose1]

Instead of os.UserHomeDir() use XDG base directories: https://specifications.freedesktop.org/basedir-spec/latest/

	// ...
	trustedDir = os.Getenv("MINITRUST_DIR")
	if trustedDir == "" {
		homedir, err := os.UserHomeDir()
		if err != nil {
			logger.Fatal(err)
		}
		trustedDir = filepath.Join(homedir, ".minisign", "trusted")
	}
	// ...