Fix build-with-cabal.sh when XDG_CONFIG_HOME is defined

[gabrielsimoes]

Description

The current script fails in case the project files are in $XDG_CONFIG_HOME/xmonad/, it tries to read them from $XDG_CONFIG_HOME directly.

Checklist

• [x] I've read CONTRIBUTING.md

• [x] I've considered how to best test these changes (property, unit, manually, ...) and concluded: manually

• [x] I updated the CHANGES.md file

[slotThe]

Thanks!