ci: Symlink gir directory to ourselves to find generator.py
ci: Symlink gir directory to ourselves to find generator.py

`generator.py` moved into this repository, and gtk-rs (and friends) now
symlink to `gir/generator.py`.  We could simply call `../generator.py`
instead, but this make sure the symlink is actually valid.

GuillaumeGomez/gir,GuillaumeGomez/gir,gtk-rs/gir,gtk-rs/gir,gtk-rs/gir,GuillaumeGomez/gir