Share gnome-build-meta runners with gnome/glib namespace

I was working on a ci job for glib based on the build-meta CI setup so that we can catch regressions that stop other projects from building, which has been fairly frequent lately. To do that and test it we need special runners though as they need to be able to run bwrap inside the docker container.

https://gitlab.gnome.org/alatiera/glib/-/jobs/2505614

My plan is to either the job being executed manually/right before the MR is merged and its assigned to marge (if that can work with the merge-request pipeline access) and/or on post-merge pipelines. So we avoid killing the runners with load.

Sharing the build-meta runners with gnome/glib should be enough to test it out and avoid excessive use, can only do x86 for the time being though aarch64 might be nice to have later on.