fix: check XDG_DATA_DIR for macos helper

• Add XDG_DATA_DIRS variable to the MacOS Helper Script
• Add MacOS build files to .gitignore

Problem

When using ghostty, the MacOS script in its current configuration fails to launch Gajim.
This is because ghostty overrides XDG_DATA_DIRS.

Adding XDG_DATA_DIRS is also helpful in cases where a user does not have the variable set in their shell config.

Screenshot

The screenshot shows the behavior of starting Gajim on MacOS before updating this script and after updating the script.