Configure export builds with installed gdUnit4 plugin

[MikeSchulze]

In short, if Godot crashes, then it's a problem with the Godot engine and not with GdUnit4 ;)
godotengine/godot#99284
could be related to godotengine/godot#100260

Godot has issues to open a project without the cache folder .godot and installed plugins, that typical not exists in a CI pipeline.
You need to compile the plugin before running the export.
You can do this manually by using $GODOT_BIN -path ./ -e --headless --quit-after 200
But it does not help for the export.

I suggest excluding the gdunit4 add-on.