CI: Build newer GLib in debug Docker image

We are currently falling between the cracks into a situation where the responsibility of building the GLib GIRs is being transferred from gobject-introspection to GLib. On the Docker image we have a g-i that is new enough to require a newer GLib, which the build pulls in via a subproject. However, building GIRs in GLib-as-a-subproject is not yet supported, so the GIRs don't get built properly.

We solve this by building a new enough GLib and installing it on the system, so it won't be pulled in as a subproject.

The non-debug image doesn't need to be updated because there we just use the system's gobject-introspection.