set config_dir properly when XDG_CONFIG_HOME is set

[Cj-bc]

Context

With current implementation, when XDG_CONFIG_HOME is defined (regardless its value) ./zen-release generate-config will generate config at $XDG_CONFIG_HOME/.config/zen-desktop/config.toml.

Actually, $XDG_CONFIG_HOME should contains .config as part of it according to Freedesktop's document:

$XDG_CONFIG_HOME defines the base directory relative to which user-specific
configuration files should be stored.
If $XDG_CONFIG_HOME is either not set or empty, a default equal to
$HOME/.config should be used.

from: https://specifications.freedesktop.org/basedir-spec/latest/ar01s03.html

Therefore, previous code generates config file at e.g.
~/.config/.config/zen-desktop/config.toml when XDG_CONFIG_HOME is defined, which isn't expected.

As Implementation of zen/config/config-parser.c of zwin-project/zen repository reads config file at ~/.config/zen-desktop/config.toml, it couldn't read config file at all.

Reproduce the problem

run

$ export XDG_CONFIG_HOME="$HOME/.config"
$ ./zen-release build deps

It'll generate $HOME/.config/.config/zen-desktop/config.toml

Summary

Append .config/ only if $XDG_CONFIG_HOME isn't defined.

How to check the behavior

run

$ export XDG_CONFIG_HOME="$HOME/.config"
$ ./zen-release build deps

Then now it should generate $HOME/.config/zen-desktop/config.toml as expected.