Fix path for config directory on Linux

[lcheylus]

With commit 1d26743, on Linux config file must be now read in $HOME/.config/heimdall/ directory.

But there is an issue in this commit => config directory is $HOME/.config/.heimdall/ (hidden directory on Linux).

heimdall/commons/const.go

Lines 12 to 18 in 1d26743

	var DefaultConfFolderFunc = func() string {
	home, _ := os.UserConfigDir()
	if runtime.GOOS == "darwin" {
	home = os.Getenv("HOME")
	}
	return home + "/.heimdall/"
	}

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Fix path for config folder on Linux in commons/const.go