ci: Install gi-docgen from main

In order to use the latest features it has

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3538>

.gitlab-ci.yml

@@ -101,7 +101,6 @@ variables:
       qemu-system-x86-core
       busybox
       zenity
-      gi-docgen
       python3-dbusmock
       "pkgconfig(libgcrypt)"
       "pkgconfig(libdisplay-info)"
@@ -142,6 +141,10 @@ variables:
         https://gitlab.freedesktop.org/wayland/wayland-protocols.git \
         1.33
 
+      ./.gitlab-ci/install-meson-project.sh \
+        https://gitlab.gnome.org/GNOME/gi-docgen.git \
+        main
+
       rpm -e --nodeps gnome-bluetooth-libs-devel \
                       mutter mutter-devel \
                       gnome-shell